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SubjectRe: [PATCH 2/4] filemap: kill page_cache_read usage in filemap_fault
On Fri 07-12-18 10:57:50, Jan Kara wrote:
> On Fri 30-11-18 14:58:10, Josef Bacik wrote:
> > If we do not have a page at filemap_fault time we'll do this weird
> > forced page_cache_read thing to populate the page, and then drop it
> > again and loop around and find it. This makes for 2 ways we can read a
> > page in filemap_fault, and it's not really needed. Instead add a
> > FGP_FOR_MMAP flag so that pagecache_get_page() will return a unlocked
> > page that's in pagecache. Then use the normal page locking and readpage
> > logic already in filemap_fault. This simplifies the no page in page
> > cache case significantly.
> >
> > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>
> Thanks for the patch. I like the simplification but I think it could be
> even improved... see below.
>
> > @@ -2449,9 +2426,11 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
> > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
> > ret = VM_FAULT_MAJOR;
> > retry_find:
> > - page = find_get_page(mapping, offset);
> > + page = pagecache_get_page(mapping, offset,
> > + FGP_CREAT|FGP_FOR_MMAP,
> > + vmf->gfp_mask);
> > if (!page)
> > - goto no_cached_page;
> > + return vmf_error(-ENOMEM);
>
> So why don't you just do:
>
> page = pagecache_get_page(mapping, offset,
> FGP_CREAT | FGP_LOCK, vmf->gfp_mask);
> if (!page)
> return vmf_error(-ENOMEM);
> goto check_uptodate;
>
> where check_uptodate would be a label before 'PageUptodate' check?
>
> Then you don't have to introduce new flag for pagecache_get_page() and you
> also don't have to unlock and then lock again the page... And you can still
> delete all the code you've deleted.

Ah, you don't want lock_page() to block in case someone raced with you and
instantiated the page so that you can drop mmap_sem. OK, the patch looks
good to me then. You can add:

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

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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