Re: [PATCH 04/17] mm/filemap: Support readpage splitting a page

[Christoph Hellwig <hch@lst.de>]

On Mon, Nov 02, 2020 at 06:42:59PM +0000, Matthew Wilcox (Oracle) wrote:
> For page splitting to succeed, the thread asking to split the
> page has to be the only one with a reference to the page.  Calling
> wait_on_page_locked() while holding a reference to the page will
> effectively prevent this from happening with sufficient threads waiting
> on the same page.  Use put_and_wait_on_page_locked() to sleep without
> holding a reference to the page, then retry the page lookup after the
> page is unlocked.
> 
> Since we now get the page lock a little earlier in filemap_update_page(),
> we can eliminate a number of duplicate checks.  The original intent
> (commit ebded02788b5 ("avoid unnecessary calls to lock_page when waiting
> for IO to complete during a read")) behind getting the page lock later
> was to avoid re-locking the page after it has been brought uptodate by
> another thread.  We still avoid that because we go through the normal
> lookup path again after the winning thread has brought the page uptodate.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>