From: Josef Bacik <josef@toxicpanda.com>
To: Qu Wenruo <wqu@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v4 06/18] btrfs: support subpage for extent buffer page release
Date: Wed, 20 Jan 2021 09:44:49 -0500 [thread overview]
Message-ID: <deac0208-7c23-f51b-7ff0-c661feba9806@toxicpanda.com> (raw)
In-Reply-To: <20210116071533.105780-7-wqu@suse.com>
On 1/16/21 2:15 AM, Qu Wenruo wrote:
> In btrfs_release_extent_buffer_pages(), we need to add extra handling
> for subpage.
>
> To do so, introduce a new helper, detach_extent_buffer_page(), to do
> different handling for regular and subpage cases.
>
> For subpage case, the new trick is about when to detach the page
> private.
>
> For unammped (dummy or cloned) ebs, we can detach the page private
> immediately as the page can only be attached to one unmapped eb.
>
> For mapped ebs, we have to ensure there are no eb in the page range
> before we delete it, as page->private is shared between all ebs in the
> same page.
>
> But there is a subpage specific race, where we can race with extent
> buffer allocation, and clear the page private while new eb is still
> being utilized, like this:
>
> Extent buffer A is the new extent buffer which will be allocated,
> while extent buffer B is the last existing extent buffer of the page.
>
> T1 (eb A) | T2 (eb B)
> -------------------------------+------------------------------
> alloc_extent_buffer() | btrfs_release_extent_buffer_pages()
> |- p = find_or_create_page() | |
> |- attach_extent_buffer_page() | |
> | | |- detach_extent_buffer_page()
> | | |- if (!page_range_has_eb())
> | | | No new eb in the page range yet
> | | | As new eb A hasn't yet been
> | | | inserted into radix tree.
> | | |- btrfs_detach_subpage()
> | | |- detach_page_private();
> |- radix_tree_insert() |
>
> Then we have a metadata eb whose page has no private bit.
>
> To avoid such race, we introduce a subpage metadata specific member,
> btrfs_subpage::under_alloc.
>
> In alloc_extent_buffer() we set that bit with the critical section of
> private_lock.
> So that page_range_has_eb() will return true for
> detach_extent_buffer_page(), and not to detach page private.
>
> New helpers are introduced to do the start/end work:
> - btrfs_page_start_meta_alloc()
> - btrfs_page_end_meta_alloc()
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
This is overly complicated, why not just have subpage->refs or ->attached as
eb's are attached to the subpage? Then we only detach the subpage on the last
eb that's referencing that page. This is much more straightforward and avoids a
whole other radix tree lookup on release. Thanks,
Josef
next prev parent reply other threads:[~2021-01-20 14:52 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-16 7:15 [PATCH v4 00/18] btrfs: add read-only support for subpage sector size Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 01/18] btrfs: update locked page dirty/writeback/error bits in __process_pages_contig() Qu Wenruo
2021-01-19 21:41 ` Josef Bacik
2021-01-21 6:32 ` Qu Wenruo
2021-01-21 6:51 ` Qu Wenruo
2021-01-23 19:13 ` David Sterba
2021-01-24 0:35 ` Qu Wenruo
2021-01-24 11:49 ` David Sterba
2021-01-16 7:15 ` [PATCH v4 02/18] btrfs: merge PAGE_CLEAR_DIRTY and PAGE_SET_WRITEBACK into PAGE_START_WRITEBACK Qu Wenruo
2021-01-19 21:43 ` Josef Bacik
2021-01-19 21:45 ` Josef Bacik
2021-01-16 7:15 ` [PATCH v4 03/18] btrfs: introduce the skeleton of btrfs_subpage structure Qu Wenruo
2021-01-18 22:46 ` David Sterba
2021-01-18 22:54 ` Qu Wenruo
2021-01-19 15:51 ` David Sterba
2021-01-19 16:06 ` David Sterba
2021-01-20 0:19 ` Qu Wenruo
2021-01-23 19:37 ` David Sterba
2021-01-24 0:24 ` Qu Wenruo
2021-01-18 23:01 ` David Sterba
2021-01-16 7:15 ` [PATCH v4 04/18] btrfs: make attach_extent_buffer_page() to handle subpage case Qu Wenruo
2021-01-18 22:51 ` David Sterba
2021-01-19 21:54 ` Josef Bacik
2021-01-19 22:35 ` David Sterba
2021-01-26 7:29 ` Qu Wenruo
2021-01-27 19:58 ` David Sterba
2021-01-20 0:27 ` Qu Wenruo
2021-01-20 14:22 ` Josef Bacik
2021-01-21 1:20 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 05/18] btrfs: make grab_extent_buffer_from_page() " Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 06/18] btrfs: support subpage for extent buffer page release Qu Wenruo
2021-01-20 14:44 ` Josef Bacik [this message]
2021-01-21 0:45 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 07/18] btrfs: attach private to dummy extent buffer pages Qu Wenruo
2021-01-20 14:48 ` Josef Bacik
2021-01-21 0:47 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 08/18] btrfs: introduce helper for subpage uptodate status Qu Wenruo
2021-01-19 19:45 ` David Sterba
2021-01-20 14:55 ` Josef Bacik
2021-01-26 7:21 ` Qu Wenruo
2021-01-20 15:00 ` Josef Bacik
2021-01-21 0:49 ` Qu Wenruo
2021-01-21 1:28 ` Josef Bacik
2021-01-21 1:38 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 09/18] btrfs: introduce helper for subpage error status Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 10/18] btrfs: make set/clear_extent_buffer_uptodate() to support subpage size Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 11/18] btrfs: make btrfs_clone_extent_buffer() to be subpage compatible Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 12/18] btrfs: implement try_release_extent_buffer() for subpage metadata support Qu Wenruo
2021-01-20 15:05 ` Josef Bacik
2021-01-21 0:51 ` Qu Wenruo
2021-01-23 20:36 ` David Sterba
2021-01-25 20:02 ` Josef Bacik
2021-01-16 7:15 ` [PATCH v4 13/18] btrfs: introduce read_extent_buffer_subpage() Qu Wenruo
2021-01-20 15:08 ` Josef Bacik
2021-01-16 7:15 ` [PATCH v4 14/18] btrfs: extent_io: make endio_readpage_update_page_status() to handle subpage case Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 15/18] btrfs: disk-io: introduce subpage metadata validation check Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 16/18] btrfs: introduce btrfs_subpage for data inodes Qu Wenruo
2021-01-19 20:48 ` David Sterba
2021-01-20 15:28 ` Josef Bacik
2021-01-26 7:05 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 17/18] btrfs: integrate page status update for data read path into begin/end_page_read() Qu Wenruo
2021-01-20 15:41 ` Josef Bacik
2021-01-21 1:05 ` Qu Wenruo
2021-01-16 7:15 ` [PATCH v4 18/18] btrfs: allow RO mount of 4K sector size fs on 64K page system Qu Wenruo
2021-01-18 23:17 ` [PATCH v4 00/18] btrfs: add read-only support for subpage sector size David Sterba
2021-01-18 23:26 ` Qu Wenruo
2021-01-24 12:29 ` David Sterba
2021-01-25 1:19 ` Qu Wenruo
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