Re: [PATCH v4 01/18] btrfs: update locked page dirty/writeback/error bits in __process_pages_contig()

From: David Sterba <dsterba@suse.cz>
To: Qu Wenruo <wqu@suse.com>
Cc: Qu Wenruo <quwenruo.btrfs@gmx.com>,
	Josef Bacik <josef@toxicpanda.com>,
	linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v4 01/18] btrfs: update locked page dirty/writeback/error bits in __process_pages_contig()
Date: Sat, 23 Jan 2021 20:13:04 +0100	[thread overview]
Message-ID: <20210123191304.GA1993@twin.jikos.cz> (raw)
In-Reply-To: <bab806e1-ad3d-b34c-b623-915b39621983@suse.com>

On Thu, Jan 21, 2021 at 02:51:46PM +0800, Qu Wenruo wrote:
> 
> 
> On 2021/1/21 下午2:32, Qu Wenruo wrote:
> > 
> > 
> > On 2021/1/20 上午5:41, Josef Bacik wrote:
> >> On 1/16/21 2:15 AM, Qu Wenruo wrote:
> >>> When __process_pages_contig() get called for
> >>> extent_clear_unlock_delalloc(), if we hit the locked page, only Private2
> >>> bit is updated, but dirty/writeback/error bits are all skipped.
> >>>
> >>> There are several call sites call extent_clear_unlock_delalloc() with
> >>> @locked_page and PAGE_CLEAR_DIRTY/PAGE_SET_WRITEBACK/PAGE_END_WRITEBACK
> >>>
> >>> - cow_file_range()
> >>> - run_delalloc_nocow()
> >>> - cow_file_range_async()
> >>>    All for their error handling branches.
> >>>
> >>> For those call sites, since we skip the locked page for
> >>> dirty/error/writeback bit update, the locked page will still have its
> >>> dirty bit remaining.
> >>>
> >>> Thankfully, since all those call sites can only be hit with various
> >>> serious errors, it's pretty hard to hit and shouldn't affect regular
> >>> btrfs operations.
> >>>
> >>> But still, we shouldn't leave the locked_page with its
> >>> dirty/error/writeback bits untouched.
> >>>
> >>> Fix this by only skipping lock/unlock page operations for locked_page.
> >>>
> >>> Signed-off-by: Qu Wenruo <wqu@suse.com>
> >>
> >> Except this is handled by the callers.  We clear_page_dirty_for_io() the
> >> page before calling btrfs_run_delalloc_range(), so we don't need the
> >> PAGE_CLEAR_DIRTY, it's already cleared.  The SetPageError() is handled
> >> in the error path for locked_page, as is the
> >> set_writeback/end_writeback.  Now I don't think this patch causes
> >> problems specifically, but the changelog is at least wrong, and I'd
> >> rather we'd skip the handling of the locked_page here and leave it in
> >> the proper error handling.  If you need to do this for some other reason
> >> that I haven't gotten to yet then you need to make that clear in the
> >> changelog, because as of right now I don't see why this is needed.  
> >> Thanks,
> > 
> > This is mostly to co-operate with a later patch on
> > __process_pages_contig(), where we need to make sure page locked by
> > __process_pages_contig() is only unlocked by __process_pages_contig() too.
> > 
> > The exception is after cow_file_inline(), we call
> > __process_pages_contig() on the locked page, making it to clear page
> > writeback and unlock it.
> 
> To be more clear, we call extent_clear_unlock_delalloc() with 
> locked_page == NULL, to allow __process_pages_contig() to unlock the 
> locked page (while the locked page isn't locked by 
> __process_pages_contig()).
> 
> For subpage data, we need writers accounting for subpage, but that 
> accounting only happens in __process_pages_contig(), thus we don't want 
> pages without the accounting to be unlocked by __process_pages_contig().
> 
> I can do extra page unlock/clear_dirty/end_writeback just for that 
> exception, but it would definitely needs more comments.

This is patch 1 and other depend on the changed behaviour so if it's
just updated changelog and comments, and Josef is ok with the result, I
can take it but otherwise this could delay the series once the rest is
reworked.