[PATCH v2] mm: readahead: make thp readahead conditional to mmap_miss logic

[Roman Gushchin <roman.gushchin@linux.dev>]

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.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: linux-mm@kvack.org

--

v2: fixed VM_SEQ_READ handling (by Dev Jain)
---
 mm/filemap.c | 42 ++++++++++++++++++++++--------------------
 1 file changed, 22 insertions(+), 20 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index a52dd38d2b4a..446e591d57e5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3235,37 +3235,23 @@ 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) {
-		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-		ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1);
-		ra->size = HPAGE_PMD_NR;
-		/*
-		 * Fetch two PMD folios, so we get the chance to actually
-		 * readahead, unless we've been told not to.
-		 */
-		if (!(vm_flags & VM_RAND_READ))
-			ra->size *= 2;
-		ra->async_size = HPAGE_PMD_NR;
-		ra->order = HPAGE_PMD_ORDER;
-		page_cache_ra_order(&ractl, ra);
-		return fpin;
-	}
-#endif
-
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+	    (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER)
+		force_thp_readahead = true;
 	/*
 	 * 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)
+	if (!ra->ra_pages && !force_thp_readahead)
 		return fpin;
 
-	if (vm_flags & VM_SEQ_READ) {
+	if ((vm_flags & VM_SEQ_READ) && !force_thp_readahead) {
 		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
 		page_cache_sync_ra(&ractl, ra->ra_pages);
 		return fpin;
@@ -3283,6 +3269,22 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	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;
+		/*
+		 * Fetch two PMD folios, so we get the chance to actually
+		 * readahead, unless we've been told not to.
+		 */
+		if (!(vm_flags & VM_RAND_READ))
+			ra->size *= 2;
+		ra->async_size = HPAGE_PMD_NR;
+		ra->order = HPAGE_PMD_ORDER;
+		page_cache_ra_order(&ractl, ra);
+		return fpin;
+	}
+
 	if (vm_flags & VM_EXEC) {
 		/*
 		 * Allow arch to request a preferred minimum folio order for
-- 
2.51.0