On Wed, Apr 02, 2025 at 02:24:45PM +0200, Michal Hocko wrote: > On Wed 02-04-25 22:32:14, Dave Chinner wrote: > > Have a look at xlog_kvmalloc() in XFS. It implements a basic > > fast-fail, no retry high order kmalloc before it falls back to > > vmalloc by turning off direct reclaim for the kmalloc() call. > > Hence if the there isn't a high-order page on the free lists ready > > to allocate, it falls back to vmalloc() immediately. > > > > For XFS, using xlog_kvmalloc() reduced the high-order per-allocation > > overhead by around 80% when compared to a standard kvmalloc() > > call. Numbers and profiles were documented in the commit message > > (reproduced in whole below)... > > Btw. it would be really great to have such concerns to be posted to the > linux-mm ML so that we are aware of that. I have brought it up in the past, along with all the other kvmalloc API problems that are mentioned in that commit message. Unfortunately, discussion focus always ended up on calling context and API flags (e.g. whether stuff like GFP_NOFS should be supported or not) no the fast-fail-then-no-fail behaviour we need. Yes, these discussions have resulted in API changes that support some new subset of gfp flags, but the performance issues have never been addressed... > kvmalloc currently doesn't support GFP_NOWAIT semantic but it does allow > to express - I prefer SLAB allocator over vmalloc. The conditional use of __GFP_NORETRY for the kmalloc call is broken if we try to use __GFP_NOFAIL with kvmalloc() - this causes the gfp mask to hold __GFP_NOFAIL | __GFP_NORETRY.... We have a hard requirement for xlog_kvmalloc() to provide __GFP_NOFAIL semantics. IOWs, we need kvmalloc() to support kmalloc(GFP_NOWAIT) for performance with fallback to vmalloc(__GFP_NOFAIL) for correctness... > I think we could make > the default kvmalloc slab path weaker by default as those who really > want slab already have means to achieve that. There is a risk of long > term fragmentation but I think this is worth trying We've been doing this for a few years now in XFS in a hot path that can make in the order of a million xlog_kvmalloc() calls a second. We've not seen any evidence that this causes or exacerbates memory fragmentation.... -Dave. -- Dave Chinner david@xxxxxxxxxxxxx
