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 Cc: Matthew Wilcox (Oracle) Cc: Jan Kara Cc: Dev Jain Cc: linux-mm@kvack.org -- v3: fixed VM_SEQ_READ handling for the THP case (by Jan Kara) v2: fixed VM_SEQ_READ handling (by Dev Jain) --- mm/filemap.c | 68 +++++++++++++++++++++++++++++----------------------- 1 file changed, 38 insertions(+), 30 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index a52dd38d2b4a..ec731ac05551 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3235,11 +3235,47 @@ 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; @@ -3254,34 +3290,6 @@ 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) { /* -- 2.51.0