Re: [PATCH 2/4] mm: bypass mmap_miss heuristic for VM_EXEC readahead
From: Jan Kara <jack@suse.cz>
Date: 2026-03-18 16:43:56
Also in:
linux-fsdevel, linux-mm, lkml
On Tue 10-03-26 07:51:15, Usama Arif wrote:
The mmap_miss counter in do_sync_mmap_readahead() tracks whether
readahead is useful for mmap'd file access. It is incremented by 1 on
every page cache miss in do_sync_mmap_readahead(), and decremented in
two places:
- filemap_map_pages(): decremented by N for each of N pages
successfully mapped via fault-around (pages found already in cache,
evidence readahead was useful). Only pages not in the workingset
count as hits.
- do_async_mmap_readahead(): decremented by 1 when a page with
PG_readahead is found in cache.
When the counter exceeds MMAP_LOTSAMISS (100), all readahead is
disabled, including the targeted VM_EXEC readahead [1] that requests
arch-preferred folio orders for contpte mapping.
On arm64 with 64K base pages, both decrement paths are inactive:
1. filemap_map_pages() is never called because fault_around_pages
(65536 >> PAGE_SHIFT = 1) disables should_fault_around(), which
requires fault_around_pages > 1. With only 1 page in the
fault-around window, there is nothing "around" to map.
2. do_async_mmap_readahead() never fires for exec mappings because
exec readahead sets async_size = 0, so no PG_readahead markers
are placed.
With no decrements, mmap_miss monotonically increases past
MMAP_LOTSAMISS after 100 page faults, disabling all subsequent
exec readahead.
Fix this by moving the VM_EXEC readahead block above the mmap_miss
check. The exec readahead path is targeted. It reads a single folio at
the fault location with async_size=0, not speculative prefetch, so the
mmap_miss heuristic designed to throttle wasteful speculative readahead
should not gate it. The page would need to be faulted in regardless,
the only question is at what order.
[1] https://lore.kernel.org/all/20250430145920.3748738-6-ryan.roberts@arm.com/ (local)
Signed-off-by: Usama Arif <redacted>
I can see the problem but I'm not sure what you propose is the right fix.
If you move the VM_EXEC logic earlier, you'll effectively disable
VM_HUGEPAGE handling for VM_EXEC vmas which I don't think we want. So
shouldn't we rather disable mmap_miss logic for VM_EXEC vmas like:
if (!(vm_flags & (VM_SEQ_READ | VM_EXEC))) {
...
}
Honza
quoted hunk ↗ jump to hunk
--- mm/filemap.c | 72 ++++++++++++++++++++++++++++------------------------ 1 file changed, 39 insertions(+), 33 deletions(-)diff --git a/mm/filemap.c b/mm/filemap.c index 6cd7974d4adab..c064f31ecec5a 100644 --- a/mm/filemap.c +++ b/mm/filemap.c@@ -3331,6 +3331,37 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) } } + if (vm_flags & VM_EXEC) { + /* + * Allow arch to request a preferred minimum folio order for + * executable memory. This can often be beneficial to + * performance if (e.g.) arm64 can contpte-map the folio. + * Executable memory rarely benefits from readahead, due to its + * random access nature, so set async_size to 0. + * + * Limit to the boundaries of the VMA to avoid reading in any + * pad that might exist between sections, which would be a waste + * of memory. + * + * This is targeted readahead (one folio at the fault location), + * not speculative prefetch, so bypass the mmap_miss heuristic + * which would otherwise disable it after MMAP_LOTSAMISS faults. + */ + struct vm_area_struct *vma = vmf->vma; + unsigned long start = vma->vm_pgoff; + unsigned long end = start + vma_pages(vma); + unsigned long ra_end; + + ra->order = exec_folio_order(); + ra->start = round_down(vmf->pgoff, 1UL << ra->order); + ra->start = max(ra->start, start); + ra_end = round_up(ra->start + ra->ra_pages, 1UL << ra->order); + ra_end = min(ra_end, end); + ra->size = ra_end - ra->start; + ra->async_size = 0; + goto do_readahead; + } + if (!(vm_flags & VM_SEQ_READ)) { /* Avoid banging the cache line if not needed */ mmap_miss = READ_ONCE(ra->mmap_miss);@@ -3361,40 +3392,15 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) return fpin; } - if (vm_flags & VM_EXEC) { - /* - * Allow arch to request a preferred minimum folio order for - * executable memory. This can often be beneficial to - * performance if (e.g.) arm64 can contpte-map the folio. - * Executable memory rarely benefits from readahead, due to its - * random access nature, so set async_size to 0. - * - * Limit to the boundaries of the VMA to avoid reading in any - * pad that might exist between sections, which would be a waste - * of memory. - */ - struct vm_area_struct *vma = vmf->vma; - unsigned long start = vma->vm_pgoff; - unsigned long end = start + vma_pages(vma); - unsigned long ra_end; - - ra->order = exec_folio_order(); - ra->start = round_down(vmf->pgoff, 1UL << ra->order); - ra->start = max(ra->start, start); - ra_end = round_up(ra->start + ra->ra_pages, 1UL << ra->order); - ra_end = min(ra_end, end); - ra->size = ra_end - ra->start; - ra->async_size = 0; - } else { - /* - * mmap read-around - */ - ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2); - ra->size = ra->ra_pages; - ra->async_size = ra->ra_pages / 4; - ra->order = 0; - } + /* + * mmap read-around + */ + ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2); + ra->size = ra->ra_pages; + ra->async_size = ra->ra_pages / 4; + ra->order = 0; +do_readahead: fpin = maybe_unlock_mmap_for_io(vmf, fpin); ractl._index = ra->start; page_cache_ra_order(&ractl, ra);-- 2.47.3
-- Jan Kara [off-list ref] SUSE Labs, CR