mm: readahead: make thp readahead conditional to mmap_miss logic

Commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
introduced a special handling for VM_HUGEPAGE mappings: even if the
readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are allocated.

This change causes a significant regression for containers with a tight
memory.max limit, if VM_HUGEPAGE is widely used.  Prior to this commit,
mmap_miss logic would eventually lead to the readahead disablement,
effectively reducing the memory pressure in the cgroup.  With this change
the kernel is trying to allocate 1-2 huge pages for each fault, no matter
if these pages are used or not before being evicted, increasing the memory
pressure multi-fold.

To fix the regression, let's make the new VM_HUGEPAGE conditional to the
mmap_miss check, but keep independent from the ra->ra_pages.  This way the
main intention of commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE
for file mappings") stays intact, but the regression is resolved.

The logic behind this changes is simple: even if a user explicitly
requests using huge pages to back the file mapping (using VM_HUGEPAGE
flag), under a very strong memory pressure it's better to fall back to
ordinary pages.

Link: https://lkml.kernel.org/r/20251006175106.377411-1-roman.gushchin@linux.dev
Fixes: 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings")
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index 13f0259d993c9c0431e51b22a42fe0f644a4546a..893ba49808b783ac8a6dab7f00a7153a847af6a7 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3253,11 +3253,47 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
        DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
        struct file *fpin = NULL;
        vm_flags_t vm_flags = vmf->vma->vm_flags;
+       bool force_thp_readahead = false;
        unsigned short mmap_miss;
 
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
        /* Use the readahead code, even if readahead is disabled */
-       if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
+       if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+           (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER)
+               force_thp_readahead = true;
+
+       if (!force_thp_readahead) {
+               /*
+                * If we don't want any read-ahead, don't bother.
+                * VM_EXEC case below is already intended for random access.
+                */
+               if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
+                       return fpin;
+
+               if (!ra->ra_pages)
+                       return fpin;
+
+               if (vm_flags & VM_SEQ_READ) {
+                       fpin = maybe_unlock_mmap_for_io(vmf, fpin);
+                       page_cache_sync_ra(&ractl, ra->ra_pages);
+                       return fpin;
+               }
+       }
+
+       if (!(vm_flags & VM_SEQ_READ)) {
+               /* Avoid banging the cache line if not needed */
+               mmap_miss = READ_ONCE(ra->mmap_miss);
+               if (mmap_miss < MMAP_LOTSAMISS * 10)
+                       WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
+
+               /*
+                * Do we miss much more than hit in this file? If so,
+                * stop bothering with read-ahead. It will only hurt.
+                */
+               if (mmap_miss > MMAP_LOTSAMISS)
+                       return fpin;
+       }
+
+       if (force_thp_readahead) {
                fpin = maybe_unlock_mmap_for_io(vmf, fpin);
                ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
                ra->size = HPAGE_PMD_NR;
@@ -3272,34 +3308,6 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
                page_cache_ra_order(&ractl, ra);
                return fpin;
        }
-#endif
-
-       /*
-        * If we don't want any read-ahead, don't bother. VM_EXEC case below is
-        * already intended for random access.
-        */
-       if ((vm_flags & (VM_RAND_READ | VM_EXEC)) == VM_RAND_READ)
-               return fpin;
-       if (!ra->ra_pages)
-               return fpin;
-
-       if (vm_flags & VM_SEQ_READ) {
-               fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-               page_cache_sync_ra(&ractl, ra->ra_pages);
-               return fpin;
-       }
-
-       /* Avoid banging the cache line if not needed */
-       mmap_miss = READ_ONCE(ra->mmap_miss);
-       if (mmap_miss < MMAP_LOTSAMISS * 10)
-               WRITE_ONCE(ra->mmap_miss, ++mmap_miss);
-
-       /*
-        * Do we miss much more than hit in this file? If so,
-        * stop bothering with read-ahead. It will only hurt.
-        */
-       if (mmap_miss > MMAP_LOTSAMISS)
-               return fpin;
 
        if (vm_flags & VM_EXEC) {
                /*