> > Hi Jens, > > On Mon, Jun 02, 2025 at 02:54:01PM -0600, Jens Axboe wrote: > > On 6/2/25 9:49 AM, Jens Axboe wrote: > > > On 6/2/25 9:00 AM, Christoph Hellwig wrote: > > >> Hi Jens, > > >> > > >> I just tried to reference RWF_DONTCACHE semantics in a standards > > >> discussion, but it doesn't seem to be documented in the man pages > > >> or in fact anywhere else I could easily find. Could you please write > > >> up the semantics for the preadv2/pwritev2 man page? > > > > > > Sure, I can write up something for the man page. > > > > Adding Darrick as well, as a) he helped review the patches, and b) his > > phrasing is usually much better than mine. > > > > Anyway, here's my first attempt: > > > > diff --git a/man/man2/readv.2 b/man/man2/readv.2 > > index c3b0a7091619..2e23e2f15cf4 100644 > > --- a/man/man2/readv.2 > > +++ b/man/man2/readv.2 > > @@ -301,6 +301,28 @@ or their equivalent flags and system calls are used > > .B RWF_SYNC > > is specified for > > .BR pwritev2 ()). > > +.TP > > +.BR RWF_DONTCACHE " (since Linux 6.14)" > > +Reads or writes to a regular file will prune instantiated page cache content > > +when the operation completes. This is different than normal buffered I/O, > > Please use semantic newlines, even for drafts; it makes editing later > much easier. See man-pages(7): > > $ MANWIDTH=72 man man-pages | sed -n '/Use semantic newlines/,/^$/p' > Use semantic newlines > In the source of a manual page, new sentences should be started on > new lines, long sentences should be split into lines at clause > breaks (commas, semicolons, colons, and so on), and long clauses > should be split at phrase boundaries. This convention, sometimes > known as "semantic newlines", makes it easier to see the effect of > patches, which often operate at the level of individual sentences, > clauses, or phrases. > > And a quote from Brian W. Kernighan about preparing documents: > > Brian W. Kernighan, 1974 [UNIX For Beginners]: > > [ > Hints for Preparing Documents > > Most documents go through several versions > (always more than you expected) > before they are finally finished. > Accordingly, > you should do whatever possible > to make the job of changing them easy. > > First, > when you do the purely mechanical operations of typing, > type so subsequent editing will be easy. > Start each sentence on a new line. > Make lines short, > and break lines at natural places, > such as after commas and semicolons, > rather than randomly. > Since most people change documents > by rewriting phrases and > adding, deleting and rearranging sentences, > these precautions simplify any editing you have to do later. > ] > > > +where the data usually remains in cache until such time that it gets reclaimed > > +due to memory pressure. If ranges of the read or written I/O was already in > > s/was/were/ > > > +cache before this read or write, then those range will not be pruned at I/O > > s/range/&s/ > > > +completion time. Additionally, any range dirtied by a write operation with > > +.B RWF_DONTCACHE > > +set will get kicked off for writeback. This is similar to calling > > +.BR sync_file_range (2) > > +with > > +.IR SYNC_FILE_RANGE_WRITE > > +to start writeback on the given range. > > +.B RWF_DONTCACHE > > +is a hint, or best effort, where no hard guarantees are given on the state of > > +the page cache once the operation completes. > > > > +Note: file systems must support > > +this feature as well. > > I'd remove the sentence above. It's redundant with the following one. > Also, to give it more visibility, and because it's not connected with > the preceding text, I'd move it to a new paragraph with '.IP'. > > Other than this comments, the text looks good to me. Thanks! I kind of walked away from this one as I didn't have time or motivation to push it forward. FWIW, if you want to massage it into submission that'd be greatly appreciated. I'm not a regular man page contributor nor do I aim to be, but I do feel like we should this feature documented. -- Jens Axboe
