huge tmpfs: fix split_huge_page() after FALLOC_FL_KEEP_SIZE

A successful shmem_fallocate() guarantees that the extent has been
reserved, even beyond i_size when the FALLOC_FL_KEEP_SIZE flag was used.
But that guarantee is broken by shmem_unused_huge_shrink()'s attempts to
split huge pages and free their excess beyond i_size; and by other uses of
split_huge_page() near i_size.

It's sad to add a shmem inode field just for this, but I did not find a
better way to keep the guarantee.  A flag to say KEEP_SIZE has been used
would be cheaper, but I'm averse to unclearable flags.  The fallocend
field is not perfect either (many disjoint ranges might be fallocated),
but good enough; and gains another use later on.

Link: https://lkml.kernel.org/r/ca9a146-3a59-6cd3-7f28-e9a044bb1052@google.com
Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 0a8499f..bfc5899 100644 (file)
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -18,6 +18,7 @@ struct shmem_inode_info {
        unsigned long           flags;
        unsigned long           alloced;        /* data pages alloced to file */
        unsigned long           swapped;        /* subtotal assigned to swap */
+       pgoff_t                 fallocend;      /* highest fallocate endindex */
        struct list_head        shrinklist;     /* shrinkable hpage inodes */
        struct list_head        swaplist;       /* chain of maybes on swap */
        struct shared_policy    policy;         /* NUMA memory alloc policy */
@@ -119,6 +120,18 @@ static inline bool shmem_file(struct file *file)
        return shmem_mapping(file->f_mapping);
 }
 
+/*
+ * If fallocate(FALLOC_FL_KEEP_SIZE) has been used, there may be pages
+ * beyond i_size's notion of EOF, which fallocate has committed to reserving:
+ * which split_huge_page() must therefore not delete.  This use of a single
+ * "fallocend" per inode errs on the side of not deleting a reservation when
+ * in doubt: there are plenty of cases when it preserves unreserved pages.
+ */
+static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
+{
+       return max(eof, SHMEM_I(inode)->fallocend);
+}
+
 extern bool shmem_charge(struct inode *inode, long pages);
 extern void shmem_uncharge(struct inode *inode, long pages);
 

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afff3ac..890fb73 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2454,11 +2454,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 
        for (i = nr - 1; i >= 1; i--) {
                __split_huge_page_tail(head, i, lruvec, list);
-               /* Some pages can be beyond i_size: drop them from page cache */
+               /* Some pages can be beyond EOF: drop them from page cache */
                if (head[i].index >= end) {
                        ClearPageDirty(head + i);
                        __delete_from_page_cache(head + i, NULL);
-                       if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
+                       if (shmem_mapping(head->mapping))
                                shmem_uncharge(head->mapping->host, 1);
                        put_page(head + i);
                } else if (!PageAnon(page)) {
@@ -2686,6 +2686,8 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
                 * head page lock is good enough to serialize the trimming.
                 */
                end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
+               if (shmem_mapping(mapping))
+                       end = shmem_fallocend(mapping->host, end);
        }
 
        /*

diff --git a/mm/shmem.c b/mm/shmem.c
index 9ef579f..e391325 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -902,6 +902,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
        if (lend == -1)
                end = -1;       /* unsigned, so actually very big */
 
+       if (info->fallocend > start && info->fallocend <= end && !unfalloc)
+               info->fallocend = start;
+
        pagevec_init(&pvec);
        index = start;
        while (index < end && find_lock_entries(mapping, index, end - 1,
@@ -2650,7 +2653,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
        struct shmem_inode_info *info = SHMEM_I(inode);
        struct shmem_falloc shmem_falloc;
-       pgoff_t start, index, end;
+       pgoff_t start, index, end, undo_fallocend;
        int error;
 
        if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
@@ -2719,6 +2722,15 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
        inode->i_private = &shmem_falloc;
        spin_unlock(&inode->i_lock);
 
+       /*
+        * info->fallocend is only relevant when huge pages might be
+        * involved: to prevent split_huge_page() freeing fallocated
+        * pages when FALLOC_FL_KEEP_SIZE committed beyond i_size.
+        */
+       undo_fallocend = info->fallocend;
+       if (info->fallocend < end)
+               info->fallocend = end;
+
        for (index = start; index < end; ) {
                struct page *page;
 
@@ -2733,6 +2745,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
                else
                        error = shmem_getpage(inode, index, &page, SGP_FALLOC);
                if (error) {
+                       info->fallocend = undo_fallocend;
                        /* Remove the !PageUptodate pages we added */
                        if (index > start) {
                                shmem_undo_range(inode,