[LSM/MM/BPF TOPIC] Do not hold mmap_lock following folio_lock failure in page faults

[Barry Song <21cnbao@gmail.com>]

Currently, page faults use per-VMA locks whenever possible.
However, when a page fault encounters an I/O read (e.g., in
filemap_fault() or do_swap_page()), the handler releases the
per-VMA lock and requests a retry after failing to acquire
folio_lock. On the retry, the fault handler always acquires
the mmap_lock in read mode unconditionally.

This can occur frequently and may sometimes cause UI jank due
to mmap_lock contention. A proposal suggests using the
per-VMA lock in the page fault retry path instead of taking
mmap_lock [1]. Oven reported that this can significantly
improve performance by reducing mmap_lock wait time [2].

Matthew appears to suggest removing the page fault retry
entirely and instead waiting for I/O while holding the
per-VMA lock, noting that folios under I/O read may be
reclaimed by the LRU under severe memory pressure and
with thousands of threads. As a result, folios might be
reallocated, causing the page fault to repeatedly re-enter [3].
However, holding the per-VMA lock during I/O wait raises
some concerns: writers could experience long delays.

On the other hand, retrying with the per-VMA lock may reduce
mmap_lock contention, potentially shortening page fault retry
time and giving the LRU fewer opportunities to reclaim folios
waiting for I/O. By the time of the LSF/MM/BPF discussion, I
may have already created similar cases and collected
additional data.

Another potential optimization is to remove the page fault retry
without risking long I/O waits blocking writers. This occurs when
folio_lock fails but the folio is already up-to-date. In this case,
a parallel page fault may simply wait for the simultaneous PF to
complete the PTE mapping.

I would appreciate feedback from LSF/MM/BPF on approaches
to eliminate mmap_lock acquisition in page faults triggered
by folio_lock failure, for both I/O wait and PTE-mapping
wait cases, for mainline inclusion.

[1] https://lore.kernel.org/linux-mm/20251127011438.6918-1-21cnbao@gmail.com/
[2] https://lore.kernel.org/linux-mm/cccf352a-1a68-430d-83fa-a14bb5e37464@oppo.com/
[3] https://lore.kernel.org/linux-mm/aSip2mWX13sqPW_l@casper.infradead.org/

Thanks
Barry