On 8/28/25 12:22 PM, Jeff Layton wrote: > On Wed, 2025-08-27 at 15:41 -0400, Mike Snitzer wrote: >> On Wed, Aug 27, 2025 at 11:34:03AM -0400, Chuck Lever wrote: >>> On 8/26/25 2:57 PM, Mike Snitzer wrote: >>>> If NFSD_IO_DIRECT is used, expand any misaligned READ to the next >>>> DIO-aligned block (on either end of the READ). The expanded READ is >>>> verified to have proper offset/len (logical_block_size) and >>>> dma_alignment checking. >>>> >>>> Must allocate and use a bounce-buffer page (called 'start_extra_page') >>>> if/when expanding the misaligned READ requires reading extra partial >>>> page at the start of the READ so that its DIO-aligned. Otherwise that >>>> extra page at the start will make its way back to the NFS client and >>>> corruption will occur. As found, and then this fix of using an extra >>>> page verified, using the 'dt' utility: >>>> dt of=/mnt/share1/dt_a.test passes=1 bs=47008 count=2 \ >>>> iotype=sequential pattern=iot onerr=abort oncerr=abort >>>> see: https://github.com/RobinTMiller/dt.git >>>> >>>> Any misaligned READ that is less than 32K won't be expanded to be >>>> DIO-aligned (this heuristic just avoids excess work, like allocating >>>> start_extra_page, for smaller IO that can generally already perform >>>> well using buffered IO). >>>> >>>> Suggested-by: Jeff Layton <jlayton@xxxxxxxxxx> >>>> Suggested-by: Chuck Lever <chuck.lever@xxxxxxxxxx> >>>> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> >>>> Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx> >>>> --- >>>> fs/nfsd/vfs.c | 200 +++++++++++++++++++++++++++++++++++-- >>>> include/linux/sunrpc/svc.h | 5 +- >>>> 2 files changed, 194 insertions(+), 11 deletions(-) >>>> >>>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c >>>> index c340708fbab4d..64732dc8985d6 100644 >>>> --- a/fs/nfsd/vfs.c >>>> +++ b/fs/nfsd/vfs.c >>>> @@ -19,6 +19,7 @@ >>>> #include <linux/splice.h> >>>> #include <linux/falloc.h> >>>> #include <linux/fcntl.h> >>>> +#include <linux/math.h> >>>> #include <linux/namei.h> >>>> #include <linux/delay.h> >>>> #include <linux/fsnotify.h> >>>> @@ -1073,6 +1074,153 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err); >>>> } >>>> >>>> +struct nfsd_read_dio { >>>> + loff_t start; >>>> + loff_t end; >>>> + unsigned long start_extra; >>>> + unsigned long end_extra; >>>> + struct page *start_extra_page; >>>> +}; >>>> + >>>> +static void init_nfsd_read_dio(struct nfsd_read_dio *read_dio) >>>> +{ >>>> + memset(read_dio, 0, sizeof(*read_dio)); >>>> + read_dio->start_extra_page = NULL; >>>> +} >>>> + >>>> +#define NFSD_READ_DIO_MIN_KB (32 << 10) >>>> + >>>> +static bool nfsd_analyze_read_dio(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> + struct nfsd_file *nf, loff_t offset, >>>> + unsigned long len, unsigned int base, >>>> + struct nfsd_read_dio *read_dio) >>>> +{ >>>> + const u32 dio_blocksize = nf->nf_dio_read_offset_align; >>>> + loff_t middle_end, orig_end = offset + len; >>>> + >>>> + if (WARN_ONCE(!nf->nf_dio_mem_align || !nf->nf_dio_read_offset_align, >>>> + "%s: underlying filesystem has not provided DIO alignment info\n", >>>> + __func__)) >>>> + return false; >>>> + if (WARN_ONCE(dio_blocksize > PAGE_SIZE, >>>> + "%s: underlying storage's dio_blocksize=%u > PAGE_SIZE=%lu\n", >>>> + __func__, dio_blocksize, PAGE_SIZE)) >>>> + return false; >>> >>> IMHO these checks do not warrant a WARN. Perhaps a trace event, instead? >> >> I won't die on this hill, I just don't see the risk of these given >> they are highly unlikely ("famous last words"). >> >> But if they trigger we should surely be made aware immediately. Not >> only if someone happens to have a trace event enabled (which would >> only happen with further support and engineering involvement to chase >> "why isn't O_DIRECT being used like NFSD was optionally configured >> to!?"). >> > > > A kernel log message in this case makes sense to me, since it is a > (minor) administrative issue. WARN_ONCE() is going to throw a big, > scary stack trace, however that won't be terribly useful. We'll get hit > with bug reports from it for years. Agreed, the stack trace isn't very useful information. > Maybe pr_notice_once() for this? Or, maybe a pr_notice_once, but do it > on a per-export basis? Right, I think warning once and then turning off the warning for all subsequent problems is going to cause a lot of missed problems. Warning once per export sounds like a step in the right direction. >>>> + /* Return early if IO is irreparably misaligned (len < PAGE_SIZE, >>>> + * or base not aligned). >>>> + * Ondisk alignment is implied by the following code that expands >>>> + * misaligned IO to have a DIO-aligned offset and len. >>>> + */ >>>> + if (unlikely(len < dio_blocksize) || ((base & (nf->nf_dio_mem_align-1)) != 0)) >>>> + return false; >>>> + >>>> + init_nfsd_read_dio(read_dio); >>>> + >>>> + read_dio->start = round_down(offset, dio_blocksize); >>>> + read_dio->end = round_up(orig_end, dio_blocksize); >>>> + read_dio->start_extra = offset - read_dio->start; >>>> + read_dio->end_extra = read_dio->end - orig_end; >>>> + >>>> + /* >>>> + * Any misaligned READ less than NFSD_READ_DIO_MIN_KB won't be expanded >>>> + * to be DIO-aligned (this heuristic avoids excess work, like allocating >>>> + * start_extra_page, for smaller IO that can generally already perform >>>> + * well using buffered IO). >>>> + */ >>>> + if ((read_dio->start_extra || read_dio->end_extra) && >>>> + (len < NFSD_READ_DIO_MIN_KB)) { >>>> + init_nfsd_read_dio(read_dio); >>>> + return false; >>>> + } >>>> + >>>> + if (read_dio->start_extra) { >>>> + read_dio->start_extra_page = alloc_page(GFP_KERNEL); >>> >>> This introduces a page allocation where there weren't any before. For >>> NFSD, I/O pages come from rqstp->rq_pages[] so that memory allocation >>> like this is not needed on an I/O path. >> >> NFSD never supported DIO before. Yes, with this patch there is >> a single page allocation in the misaligned DIO READ path (if it >> requires reading extra before the client requested data starts). >> >> I tried to succinctly explain the need for the extra page allocation >> for misaligned DIO READ in this patch's header (in 2nd paragraph >> of the above header). >> >> I cannot see how to read extra, not requested by the client, into the >> head of rq_pages without causing serious problems. So that cannot be >> what you're saying needed. >> >>> So I think the answer to this is that I want you to implement reading >>> an entire aligned range from the file and then forming the NFS READ >>> response with only the range of bytes that the client requested, as we >>> discussed before. >> >> That is what I'm doing. But you're taking issue with my implementation >> that uses a single extra page. >> >>> The use of xdr_buf and bvec should make that quite >>> straightforward. >> >> Is your suggestion to, rather than allocate a disjoint single page, >> borrow the extra page from the end of rq_pages? Just map it into the >> bvec instead of my extra page? >> >>> This should make the aligned and unaligned cases nearly identical and >>> much less fraught. >> >> Regardless of which memory used to read the extra data, I don't see >> how the care I've taken to read extra but hide that fact from the >> client can be avoided. So the pre/post misaligned DIO READ code won't >> change a whole lot. But once I understand your suggestion better >> (after a clarifying response to this message) hopefully I'll see what >> you're saying. >> >> All said, this patchset is very important to me, I don't want it to >> miss v6.18 -- I'm still "in it to win it" but it feels like I do need >> your or others' help to pull this off. >> >> And/or is it possible to accept this initial implementation with >> mutual understanding that we must revisit your concern about my >> allocating an extra page for the misaligned DIO READ path? >> >>>> + if (WARN_ONCE(read_dio->start_extra_page == NULL, >>>> + "%s: Unable to allocate start_extra_page\n", __func__)) { >>>> + init_nfsd_read_dio(read_dio); >>>> + return false; >>>> + } >>>> + } >>>> + >>>> + return true; >>>> +} >>>> + >>>> +static ssize_t nfsd_complete_misaligned_read_dio(struct svc_rqst *rqstp, >>>> + struct nfsd_read_dio *read_dio, >>>> + ssize_t bytes_read, >>>> + unsigned long bytes_expected, >>>> + loff_t *offset, >>>> + unsigned long *rq_bvec_numpages) >>>> +{ >>>> + ssize_t host_err = bytes_read; >>>> + loff_t v; >>>> + >>>> + if (!read_dio->start_extra && !read_dio->end_extra) >>>> + return host_err; >>>> + >>>> + /* If nfsd_analyze_read_dio() allocated a start_extra_page it must >>>> + * be removed from rqstp->rq_bvec[] to avoid returning unwanted data. >>>> + */ >>>> + if (read_dio->start_extra_page) { >>>> + __free_page(read_dio->start_extra_page); >>>> + *rq_bvec_numpages -= 1; >>>> + v = *rq_bvec_numpages; >>>> + memmove(rqstp->rq_bvec, rqstp->rq_bvec + 1, >>>> + v * sizeof(struct bio_vec)); >>>> + } >>>> + /* Eliminate any end_extra bytes from the last page */ >>>> + v = *rq_bvec_numpages; >>>> + rqstp->rq_bvec[v].bv_len -= read_dio->end_extra; >>>> + >>>> + if (host_err < 0) { >>>> + /* Underlying FS will return -EINVAL if misaligned >>>> + * DIO is attempted because it shouldn't be. >>>> + */ >>>> + WARN_ON_ONCE(host_err == -EINVAL); >>>> + return host_err; >>>> + } >>>> + >>>> + /* nfsd_analyze_read_dio() may have expanded the start and end, >>>> + * if so adjust returned read size to reflect original extent. >>>> + */ >>>> + *offset += read_dio->start_extra; >>>> + if (likely(host_err >= read_dio->start_extra)) { >>>> + host_err -= read_dio->start_extra; >>>> + if (host_err > bytes_expected) >>>> + host_err = bytes_expected; >>>> + } else { >>>> + /* Short read that didn't read any of requested data */ >>>> + host_err = 0; >>>> + } >>>> + >>>> + return host_err; >>>> +} >>>> + >>>> +static bool nfsd_iov_iter_aligned_bvec(const struct iov_iter *i, >>>> + unsigned addr_mask, unsigned len_mask) >>>> +{ >>>> + const struct bio_vec *bvec = i->bvec; >>>> + unsigned skip = i->iov_offset; >>>> + size_t size = i->count; >>> >>> checkpatch.pl is complaining about the use of "unsigned" rather than >>> "unsigned int". >> >> OK. >> >>>> + >>>> + if (size & len_mask) >>>> + return false; >>>> + do { >>>> + size_t len = bvec->bv_len; >>>> + >>>> + if (len > size) >>>> + len = size; >>>> + if ((unsigned long)(bvec->bv_offset + skip) & addr_mask) >>>> + return false; >>>> + bvec++; >>>> + size -= len; >>>> + skip = 0; >>>> + } while (size); >>>> + >>>> + return true; >>>> +} >>>> + >>>> /** >>>> * nfsd_iter_read - Perform a VFS read using an iterator >>>> * @rqstp: RPC transaction context >>>> @@ -1094,7 +1242,8 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> unsigned int base, u32 *eof) >>>> { >>>> struct file *file = nf->nf_file; >>>> - unsigned long v, total; >>>> + unsigned long v, total, in_count = *count; >>>> + struct nfsd_read_dio read_dio; >>>> struct iov_iter iter; >>>> struct kiocb kiocb; >>>> ssize_t host_err; >>>> @@ -1102,13 +1251,34 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> >>>> init_sync_kiocb(&kiocb, file); >>>> >>>> + v = 0; >>>> + total = in_count; >>>> + >>>> switch (nfsd_io_cache_read) { >>>> case NFSD_IO_DIRECT: >>>> - /* Verify ondisk and memory DIO alignment */ >>>> - if (nf->nf_dio_mem_align && nf->nf_dio_read_offset_align && >>>> - (((offset | *count) & (nf->nf_dio_read_offset_align - 1)) == 0) && >>>> - (base & (nf->nf_dio_mem_align - 1)) == 0) >>>> - kiocb.ki_flags = IOCB_DIRECT; >>>> + /* >>>> + * If NFSD_IO_DIRECT enabled, expand any misaligned READ to >>>> + * the next DIO-aligned block (on either end of the READ). >>>> + */ >>>> + if (nfsd_analyze_read_dio(rqstp, fhp, nf, offset, >>>> + in_count, base, &read_dio)) { >>>> + /* trace_nfsd_read_vector() will reflect larger >>>> + * DIO-aligned READ. >>>> + */ >>>> + offset = read_dio.start; >>>> + in_count = read_dio.end - offset; >>>> + total = in_count; >>>> + >>>> + kiocb.ki_flags |= IOCB_DIRECT; >>>> + if (read_dio.start_extra) { >>>> + len = read_dio.start_extra; >>>> + bvec_set_page(&rqstp->rq_bvec[v], >>>> + read_dio.start_extra_page, >>>> + len, PAGE_SIZE - len); >>>> + total -= len; >>>> + ++v; >>>> + } >>>> + } >>>> break; >>>> case NFSD_IO_DONTCACHE: >>>> kiocb.ki_flags = IOCB_DONTCACHE; >>>> @@ -1120,8 +1290,6 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> >>>> kiocb.ki_pos = offset; >>>> >>>> - v = 0; >>>> - total = *count; >>>> while (total) { >>>> len = min_t(size_t, total, PAGE_SIZE - base); >>>> bvec_set_page(&rqstp->rq_bvec[v], *(rqstp->rq_next_page++), >>>> @@ -1132,9 +1300,21 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp, >>>> } >>>> WARN_ON_ONCE(v > rqstp->rq_maxpages); >>>> >>>> - trace_nfsd_read_vector(rqstp, fhp, offset, *count); >>>> - iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count); >>>> + trace_nfsd_read_vector(rqstp, fhp, offset, in_count); >>>> + iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, in_count); >>>> + >>>> + if ((kiocb.ki_flags & IOCB_DIRECT) && >>>> + !nfsd_iov_iter_aligned_bvec(&iter, nf->nf_dio_mem_align-1, >>>> + nf->nf_dio_read_offset_align-1)) >>>> + kiocb.ki_flags &= ~IOCB_DIRECT; >>>> + >>>> host_err = vfs_iocb_iter_read(file, &kiocb, &iter); >>>> + >>>> + if (in_count != *count) { >>>> + /* misaligned DIO expanded read to be DIO-aligned */ >>>> + host_err = nfsd_complete_misaligned_read_dio(rqstp, &read_dio, >>>> + host_err, *count, &offset, &v); >>>> + } >>>> return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err); >>>> } >>>> >>>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h >>>> index e64ab444e0a7f..190c2667500e2 100644 >>>> --- a/include/linux/sunrpc/svc.h >>>> +++ b/include/linux/sunrpc/svc.h >>>> @@ -163,10 +163,13 @@ extern u32 svc_max_payload(const struct svc_rqst *rqstp); >>>> * pages, one for the request, and one for the reply. >>>> * nfsd_splice_actor() might need an extra page when a READ payload >>>> * is not page-aligned. >>>> + * nfsd_iter_read() might need two extra pages when a READ payload >>>> + * is not DIO-aligned -- but nfsd_iter_read() and nfsd_splice_actor() >>>> + * are mutually exclusive (so reuse page reserved for nfsd_splice_actor). >>>> */ >>>> static inline unsigned long svc_serv_maxpages(const struct svc_serv *serv) >>>> { >>>> - return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1; >>>> + return DIV_ROUND_UP(serv->sv_max_mesg, PAGE_SIZE) + 2 + 1 + 1; >>>> } >>>> >>>> /* >>> >>> To properly evaluate the impact of using direct I/O for reads with real >>> world user workloads, we will want to identify (or construct) some >>> metrics (and this is future work, but near-term future). >>> >>> Seems like allocating memory becomes difficult only when too many pages >>> are dirty. I am skeptical that the issue is due to read caching, since >>> clean pages in the page cache are pretty easy to evict quickly, AIUI. If >>> that's incorrect, I'd like to understand why. >> >> The much more problematic case is heavy WRITE workload with a working >> set that far exceeds system memory. >> >> But I agree it doesn't make a whole lot of sense that clean pages in >> the page cache would be getting in the way. All I can tell you is >> that in my experience MM seems to _not_ evict them quickly (but more >> focused read-only testing is warranted to further understand the >> dynamics and heuristics in MM and beyond -- especially if/when >> READ-only then a pivot to a mix of heavy READ and WRITE or >> WRITE-only). >> >> NFSD using DIO is optional. I thought the point was to get it as an >> available option so that _others_ could experiment and help categorize >> the benefits/pitfalls further? >> >> I cannot be a one man show on all this. I welcome more help from >> anyone interested. > -- Chuck Lever
