Re: [PATCH 3/8] shmem: factor shmem_falloc_wait() out of shmem_fault()

Jan Kara
2023-10-03T13:19:04Z

On Fri 29-09-23 20:27:53, Hugh Dickins wrote:
> That Trinity livelock shmem_falloc avoidance block is unlikely, and a
> distraction from the proper business of shmem_fault(): separate it out.
> (This used to help compilers save stack on the fault path too, but both
> gcc and clang nowadays seem to make better choices anyway.)
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Looking at the code I'm just wondering whether the livelock with
shmem_undo_range() couldn't be more easy to avoid by making
shmem_undo_range() always advance the index by 1 after evicting a page and
thus guaranteeing a forward progress... Because the forward progress within
find_get_entries() is guaranteed these days, it should be enough.

Honza

> ---
> mm/shmem.c | 126 +++++++++++++++++++++++++++++------------------------
> 1 file changed, 69 insertions(+), 57 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 824eb55671d2..5501a5bc8d8c 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2148,87 +2148,99 @@ int shmem_get_folio(struct inode *inode, pgoff_t index, struct folio **foliop,
> * entry unconditionally - even if something else had already woken the
> * target.
> */
> -static int synchronous_wake_function(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
> +static int synchronous_wake_function(wait_queue_entry_t *wait,
> + unsigned int mode, int sync, void *key)
> {
> int ret = default_wake_function(wait, mode, sync, key);
> list_del_init(&wait->entry);
> return ret;
> }
>
> +/*
> + * 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_rwsem. So refrain from
> + * faulting pages into the hole while it's being punched. Although
> + * shmem_undo_range() does remove the additions, it may be unable to
> + * keep up, as each new page needs its own unmap_mapping_range() call,
> + * and the i_mmap tree grows ever slower to scan if new vmas are added.
> + *
> + * It does not matter if we sometimes reach this check just before the
> + * hole-punch begins, so that one fault then races with the punch:
> + * we just need to make racing faults a rare case.
> + *
> + * The implementation below would be much simpler if we just used a
> + * standard mutex or completion: but we cannot take i_rwsem in fault,
> + * and bloating every shmem inode for this unlikely case would be sad.
> + */
> +static vm_fault_t shmem_falloc_wait(struct vm_fault *vmf, struct inode *inode)
> +{
> + struct shmem_falloc *shmem_falloc;
> + struct file *fpin = NULL;
> + vm_fault_t ret = 0;
> +
> + spin_lock(&inode->i_lock);
> + shmem_falloc = inode->i_private;
> + if (shmem_falloc &&
> + shmem_falloc->waitq &&
> + vmf->pgoff >= shmem_falloc->start &&
> + vmf->pgoff < shmem_falloc->next) {
> + wait_queue_head_t *shmem_falloc_waitq;
> + DEFINE_WAIT_FUNC(shmem_fault_wait, synchronous_wake_function);
> +
> + ret = VM_FAULT_NOPAGE;
> + fpin = maybe_unlock_mmap_for_io(vmf, NULL);
> + shmem_falloc_waitq = shmem_falloc->waitq;
> + prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
> + TASK_UNINTERRUPTIBLE);
> + spin_unlock(&inode->i_lock);
> + schedule();
> +
> + /*
> + * shmem_falloc_waitq points into the shmem_fallocate()
> + * stack of the hole-punching task: shmem_falloc_waitq
> + * is usually invalid by the time we reach here, but
> + * finish_wait() does not dereference it in that case;
> + * though i_lock needed lest racing with wake_up_all().
> + */
> + spin_lock(&inode->i_lock);
> + finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
> + }
> + spin_unlock(&inode->i_lock);
> + if (fpin) {
> + fput(fpin);
> + ret = VM_FAULT_RETRY;
> + }
> + return ret;
> +}
> +
> static vm_fault_t shmem_fault(struct vm_fault *vmf)
> {
> - struct vm_area_struct *vma = vmf->vma;
> - struct inode *inode = file_inode(vma->vm_file);
> + struct inode *inode = file_inode(vmf->vma->vm_file);
> gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
> struct folio *folio = NULL;
> + vm_fault_t ret = 0;
> int err;
> - vm_fault_t 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_rwsem. So refrain from
> - * faulting pages into the hole while it's being punched. Although
> - * shmem_undo_range() does remove the additions, it may be unable to
> - * keep up, as each new page needs its own unmap_mapping_range() call,
> - * and the i_mmap tree grows ever slower to scan if new vmas are added.
> - *
> - * It does not matter if we sometimes reach this check just before the
> - * hole-punch begins, so that one fault then races with the punch:
> - * we just need to make racing faults a rare case.
> - *
> - * The implementation below would be much simpler if we just used a
> - * standard mutex or completion: but we cannot take i_rwsem in fault,
> - * and bloating every shmem inode for this unlikely case would be sad.
> + * prevent the hole-punch from ever completing: noted in i_private.
> */
> 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->waitq &&
> - vmf->pgoff >= shmem_falloc->start &&
> - vmf->pgoff < shmem_falloc->next) {
> - struct file *fpin;
> - wait_queue_head_t *shmem_falloc_waitq;
> - DEFINE_WAIT_FUNC(shmem_fault_wait, synchronous_wake_function);
> -
> - ret = VM_FAULT_NOPAGE;
> - fpin = maybe_unlock_mmap_for_io(vmf, NULL);
> - if (fpin)
> - ret = VM_FAULT_RETRY;
> -
> - shmem_falloc_waitq = shmem_falloc->waitq;
> - prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
> - TASK_UNINTERRUPTIBLE);
> - spin_unlock(&inode->i_lock);
> - schedule();
> -
> - /*
> - * shmem_falloc_waitq points into the shmem_fallocate()
> - * stack of the hole-punching task: shmem_falloc_waitq
> - * is usually invalid by the time we reach here, but
> - * finish_wait() does not dereference it in that case;
> - * though i_lock needed lest racing with wake_up_all().
> - */
> - spin_lock(&inode->i_lock);
> - finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
> - spin_unlock(&inode->i_lock);
> -
> - if (fpin)
> - fput(fpin);
> + ret = shmem_falloc_wait(vmf, inode);
> + if (ret)
> return ret;
> - }
> - spin_unlock(&inode->i_lock);
> }
>
> + WARN_ON_ONCE(vmf->page != NULL);
> err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, SGP_CACHE,
> gfp, vmf, &ret);
> if (err)
> return vmf_error(err);
> - if (folio)
> + if (folio) {
> vmf->page = folio_file_page(folio, vmf->pgoff);
> + ret |= VM_FAULT_LOCKED;
> + }
> return ret;
> }
>
> --
> 2.35.3
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR