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SubjectRe: [PATCH 2/7] mm: drop mmap_sem for page cache read IO submission
On Thu, Oct 18, 2018 at 04:23:13PM -0400, Josef Bacik wrote:
> From: Johannes Weiner <hannes@cmpxchg.org>
>
> Reads can take a long time, and if anybody needs to take a write lock on
> the mmap_sem it'll block any subsequent readers to the mmap_sem while
> the read is outstanding, which could cause long delays. Instead drop
> the mmap_sem if we do any reads at all.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
.....
> vm_fault_t filemap_fault(struct vm_fault *vmf)
> {
> int error;
> + struct mm_struct *mm = vmf->vma->vm_mm;
> struct file *file = vmf->vma->vm_file;
> struct address_space *mapping = file->f_mapping;
> struct file_ra_state *ra = &file->f_ra;
> struct inode *inode = mapping->host;
> pgoff_t offset = vmf->pgoff;
> + int flags = vmf->flags;

local copy of flags.

> pgoff_t max_off;
> struct page *page;
> vm_fault_t ret = 0;
> @@ -2509,27 +2540,44 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> * Do we have something in the page cache already?
> */
> page = find_get_page(mapping, offset);
> - if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
> + if (likely(page) && !(flags & FAULT_FLAG_TRIED)) {

Used here.

> /*
> * We found the page, so try async readahead before
> * waiting for the lock.
> */
> - do_async_mmap_readahead(vmf->vma, ra, file, page, offset);
> + error = do_async_mmap_readahead(vmf->vma, ra, file, page, offset, vmf->flags);

Not here.

> + if (error == -EAGAIN)
> + goto out_retry_wait;
> } else if (!page) {
> /* No page in the page cache at all */
> - do_sync_mmap_readahead(vmf->vma, ra, file, offset);
> - count_vm_event(PGMAJFAULT);
> - count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
> ret = VM_FAULT_MAJOR;
> + count_vm_event(PGMAJFAULT);
> + count_memcg_event_mm(mm, PGMAJFAULT);
> + error = do_sync_mmap_readahead(vmf->vma, ra, file, offset, vmf->flags);

or here.

(Also, the vmf is passed through to where these flags
are used, so why is it passed as a separate flag parameter?)

> + if (error == -EAGAIN)
> + goto out_retry_wait;
> retry_find:
> page = find_get_page(mapping, offset);
> if (!page)
> goto no_cached_page;
> }
>
> - if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) {
> - put_page(page);
> - return ret | VM_FAULT_RETRY;
> + if (!trylock_page(page)) {
> + if (flags & FAULT_FLAG_ALLOW_RETRY) {
> + if (flags & FAULT_FLAG_RETRY_NOWAIT)
> + goto out_retry;
> + up_read(&mm->mmap_sem);
> + goto out_retry_wait;
> + }
> + if (flags & FAULT_FLAG_KILLABLE) {

but is used here...

> + int ret = __lock_page_killable(page);
> +
> + if (ret) {
> + up_read(&mm->mmap_sem);
> + goto out_retry;
> + }
> + } else
> + __lock_page(page);
> }
>
> /* Did it get truncated? */
> @@ -2607,6 +2655,19 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> /* Things didn't work out. Return zero to tell the mm layer so. */
> shrink_readahead_size_eio(file, ra);
> return VM_FAULT_SIGBUS;
> +
> +out_retry_wait:
> + if (page) {
> + if (flags & FAULT_FLAG_KILLABLE)

and here.

Any reason for this discrepancy?

-Dave.
--
Dave Chinner
david@fromorbit.com

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