[dropping individuals, leaving only mailing lists. please don't send this kind of thing to so many people in future]
On Thu, Nov 27, 2025 at 12:22:16PM +0800, Barry Song wrote: > On Thu, Nov 27, 2025 at 12:09 PM Matthew Wilcox <[email protected]> wrote: > > > > On Thu, Nov 27, 2025 at 09:14:36AM +0800, Barry Song wrote: > > > There is no need to always fall back to mmap_lock if the per-VMA > > > lock was released only to wait for pagecache or swapcache to > > > become ready. > > > > Something I've been wondering about is removing all the "drop the MM > > locks while we wait for I/O" gunk. It's a nice amount of code removed: > > I think the point is that page fault handlers should avoid holding the VMA > lock or mmap_lock for too long while waiting for I/O. Otherwise, those > writers and readers will be stuck for a while. There's a usecase some of us have been discussing off-list for a few weeks that our current strategy pessimises. It's a process with thousands (maybe tens of thousands) of threads. It has much more mapped files than it has memory that cgroups will allow it to use. So on a page fault, we drop the vma lock, allocate a page of ram, kick off the read, sleep waiting for the folio to come uptodate, once it is return, expecting the page to still be there when we reenter filemap_fault. But it's under so much memory pressure that it's already been reclaimed by the time we get back to it. So all the threads just batter the storage re-reading data. If we don't drop the vma lock, we can insert the pages in the page table and return, maybe getting some work done before this thread is descheduled. This use case also manages to get utterly hung-up trying to do reclaim today with the mmap_lock held. SO it manifests somewhat similarly to your problem (everybody ends up blocked on mmap_lock) but it has a rather different root cause. > I agree there’s room for improvement, but merely removing the "drop the MM > locks while waiting for I/O" code is unlikely to improve performance. I'm not sure it'd hurt performance. The "drop mmap locks for I/O" code was written before the VMA locking code was written. I don't know that it's actually helping these days. > The change would be much more complex, so I’d prefer to land the current > patchset first. At least this way, we avoid falling back to mmap_lock and > causing contention or priority inversion, with minimal changes. Uh, this is an RFC patchset. I'm giving you my comment, which is that I don't think this is the right direction to go in. Any talk of "landing" these patches is extremely premature.
