From: Hugh Dickins <hughd@google.com> Subject: 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). Signed-off-by: Hugh Dickins <hughd@google.com> Reported-by: Sasha Levin <sasha.levin@oracle.com> Tested-by: Sasha Levin <sasha.levin@oracle.com> Cc: Dave Jones <davej@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> --- mm/shmem.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 51 insertions(+), 4 deletions(-) diff -puN mm/shmem.c~shmem-fix-faulting-into-a-hole-while-its-punched mm/shmem.c --- a/mm/shmem.c~shmem-fix-faulting-into-a-hole-while-its-punched +++ a/mm/shmem.c @@ -80,11 +80,12 @@ static struct vfsmount *shm_mnt; #define SHORT_SYMLINK_LEN 128 /* - * 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 * spin_lock(&inode->i_lock); shmem_falloc = inode->i_private; if (shmem_falloc && + !shmem_falloc->mode && index >= shmem_falloc->start && index < shmem_falloc->next) shmem_falloc->nr_unswapped++; @@ -1233,6 +1235,43 @@ static int shmem_fault(struct vm_area_st 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; + spin_lock(&inode->i_lock); + 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; + spin_unlock(&inode->i_lock); + /* + * 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)) { + up_read(&vma->vm_mm->mmap_sem); + mutex_lock(&inode->i_mutex); + mutex_unlock(&inode->i_mutex); + return VM_FAULT_RETRY; + } + /* cond_resched? Leave that to GUP or return to user */ + return VM_FAULT_NOPAGE; + } + } + error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret); if (error) return ((error == -ENOMEM) ? VM_FAULT_OOM : VM_FAULT_SIGBUS); @@ -1729,18 +1768,26 @@ static long shmem_fallocate(struct file mutex_lock(&inode->i_mutex); + 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; + shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT; + spin_lock(&inode->i_lock); + inode->i_private = &shmem_falloc; + spin_unlock(&inode->i_lock); + 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 */ _