Commit f00cdc6d authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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shmem: fix faulting into a hole while it's punched

Trinity finds that mmap access to a hole while it's punched from shmem
can prevent the madvise(MADV_REMOVE) or fallocate(FALLOC_FL_PUNCH_HOLE)
from completing, until the reader chooses to stop; with the puncher's
hold on i_mutex locking out all other writers until it can complete.

It appears that the tmpfs fault path is too light in comparison with its
hole-punching path, lacking an i_data_sem to obstruct it; but we don't
want to slow down the common case.

Extend shmem_fallocate()'s existing range notification mechanism, so
shmem_fault() can refrain from faulting pages into the hole while it's
punched, waiting instead on i_mutex (when safe to sleep; or repeatedly
faulting when not).

[ coding-style fixes]
Signed-off-by: default avatarHugh Dickins <>
Reported-by: default avatarSasha Levin <>
Tested-by: default avatarSasha Levin <>
Cc: Dave Jones <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent f72e7dcd
......@@ -80,11 +80,12 @@ static struct vfsmount *shm_mnt;
* shmem_fallocate and shmem_writepage communicate via inode->i_private
* (with i_mutex making sure that it has only one user at a time):
* we would prefer not to enlarge the shmem inode just for that.
* shmem_fallocate communicates with shmem_fault or shmem_writepage via
* inode->i_private (with i_mutex making sure that it has only one user at
* a time): we would prefer not to enlarge the shmem inode just for that.
struct shmem_falloc {
int mode; /* FALLOC_FL mode currently operating */
pgoff_t start; /* start of range currently being fallocated */
pgoff_t next; /* the next page offset to be fallocated */
pgoff_t nr_falloced; /* how many new pages have been fallocated */
......@@ -759,6 +760,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
shmem_falloc = inode->i_private;
if (shmem_falloc &&
!shmem_falloc->mode &&
index >= shmem_falloc->start &&
index < shmem_falloc->next)
......@@ -1233,6 +1235,44 @@ static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
int error;
int ret = VM_FAULT_LOCKED;
* Trinity finds that probing a hole which tmpfs is punching can
* prevent the hole-punch from ever completing: which in turn
* locks writers out with its hold on i_mutex. So refrain from
* faulting pages into the hole while it's being punched, and
* wait on i_mutex to be released if vmf->flags permits.
if (unlikely(inode->i_private)) {
struct shmem_falloc *shmem_falloc;
shmem_falloc = inode->i_private;
if (!shmem_falloc ||
shmem_falloc->mode != FALLOC_FL_PUNCH_HOLE ||
vmf->pgoff < shmem_falloc->start ||
vmf->pgoff >= shmem_falloc->next)
shmem_falloc = NULL;
* i_lock has protected us from taking shmem_falloc seriously
* once return from shmem_fallocate() went back up that stack.
* i_lock does not serialize with i_mutex at all, but it does
* not matter if sometimes we wait unnecessarily, or sometimes
* miss out on waiting: we just need to make those cases rare.
if (shmem_falloc) {
if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
/* cond_resched? Leave that to GUP or return to user */
error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
if (error)
return ((error == -ENOMEM) ? VM_FAULT_OOM : VM_FAULT_SIGBUS);
......@@ -1729,18 +1769,26 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
shmem_falloc.mode = mode & ~FALLOC_FL_KEEP_SIZE;
if (mode & FALLOC_FL_PUNCH_HOLE) {
struct address_space *mapping = file->f_mapping;
loff_t unmap_start = round_up(offset, PAGE_SIZE);
loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
shmem_falloc.start = unmap_start >> PAGE_SHIFT; = (unmap_end + 1) >> PAGE_SHIFT;
inode->i_private = &shmem_falloc;
if ((u64)unmap_end > (u64)unmap_start)
unmap_mapping_range(mapping, unmap_start,
1 + unmap_end - unmap_start, 0);
shmem_truncate_range(inode, offset, offset + len - 1);
/* No need to unmap again: hole-punching leaves COWed pages */
error = 0;
goto out;
goto undone;
/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
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