From: Jan Kara <jack@suse.cz>
To: Josef Bacik <josef@toxicpanda.com>
Cc: kernel-team@fb.com, hannes@cmpxchg.org,
linux-kernel@vger.kernel.org, tj@kernel.org, david@fromorbit.com,
akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
linux-btrfs@vger.kernel.org, riel@fb.com, linux-mm@kvack.org
Subject: Re: [PATCH 7/7] btrfs: drop mmap_sem in mkwrite for btrfs
Date: Thu, 25 Oct 2018 15:22:30 +0200 [thread overview]
Message-ID: <20181025132230.GD7711@quack2.suse.cz> (raw)
In-Reply-To: <20181018202318.9131-8-josef@toxicpanda.com>
On Thu 18-10-18 16:23:18, Josef Bacik wrote:
> ->page_mkwrite is extremely expensive in btrfs. We have to reserve
> space, which can take 6 lifetimes, and we could possibly have to wait on
> writeback on the page, another several lifetimes. To avoid this simply
> drop the mmap_sem if we didn't have the cached page and do all of our
> work and return the appropriate retry error. If we have the cached page
> we know we did all the right things to set this page up and we can just
> carry on.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
...
> @@ -8828,6 +8830,29 @@ vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
>
> reserved_space = PAGE_SIZE;
>
> + /*
> + * We have our cached page from a previous mkwrite, check it to make
> + * sure it's still dirty and our file size matches when we ran mkwrite
> + * the last time. If everything is OK then return VM_FAULT_LOCKED,
> + * otherwise do the mkwrite again.
> + */
> + if (vmf->flags & FAULT_FLAG_USED_CACHED) {
> + lock_page(page);
> + if (vmf->cached_size == i_size_read(inode) &&
> + PageDirty(page))
> + return VM_FAULT_LOCKED;
> + unlock_page(page);
> + }
I guess this is similar to Dave's comment: Why is i_size so special? What
makes sure that file didn't get modified between time you've prepared
cached_page and now such that you need to do the preparation again?
And if indeed metadata prepared for a page cannot change, what's so special
about it being that particular cached_page?
Maybe to phrase my objections differently: Your preparations in
btrfs_page_mkwrite() are obviously related to your filesystem metadata. So
why cannot you infer from that metadata (extent tree, whatever - I'd use
extent status tree in ext4) whether that particular file+offset is already
prepared for writing and just bail out with success in that case?
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2018-10-25 13:22 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-18 20:23 [PATCH 0/7][V3] drop the mmap_sem when doing IO in the fault path Josef Bacik
2018-10-18 20:23 ` [PATCH 1/7] mm: infrastructure for page fault page caching Josef Bacik
2018-10-18 20:23 ` [PATCH 2/7] mm: drop mmap_sem for page cache read IO submission Josef Bacik
2018-10-19 3:14 ` Dave Chinner
2018-10-18 20:23 ` [PATCH 3/7] mm: drop the mmap_sem in all read fault cases Josef Bacik
2018-10-19 3:21 ` Dave Chinner
2018-10-18 20:23 ` [PATCH 4/7] mm: use the cached page for filemap_fault Josef Bacik
2018-10-19 3:27 ` Dave Chinner
2018-10-18 20:23 ` [PATCH 5/7] mm: add a flag to indicate we used a cached page Josef Bacik
2018-10-19 3:34 ` Dave Chinner
2018-10-18 20:23 ` [PATCH 6/7] mm: allow ->page_mkwrite to do retries Josef Bacik
2018-10-19 3:36 ` Dave Chinner
2018-10-18 20:23 ` [PATCH 7/7] btrfs: drop mmap_sem in mkwrite for btrfs Josef Bacik
2018-10-19 3:48 ` Dave Chinner
2018-10-22 17:56 ` Josef Bacik
2018-10-22 21:31 ` Dave Chinner
2018-10-25 13:22 ` Jan Kara [this message]
2018-10-25 13:58 ` Josef Bacik
2018-10-26 9:47 ` Jan Kara
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