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SubjectRe: [PATCH 7/7] btrfs: drop mmap_sem in mkwrite for btrfs
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

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