[Linux-cachefs] [PATCH 14/14] mm, netfs, fscache: Stop read optimisation when folio removed from pagecache

[Matthew Wilcox]

On Thu, Apr 07, 2022 at 07:41:47AM +0100, David Howells wrote:
> Matthew Wilcox <willy at infradead.org> wrote:
> 
> > On Thu, Apr 07, 2022 at 12:05:05AM +0100, David Howells wrote:
> > > Fix this by adding an extra address_space operation, ->removing folio(),
> > > and flag, AS_NOTIFY_REMOVING_FOLIO.  The operation is called if the flag is
> > > set when a folio is removed from the pagecache.  The flag should be set if
> > > a non-NULL cookie is obtained from fscache and cleared in ->evict_inode()
> > > before truncate_inode_pages_final() is called.
> > 
> > What's wrong with ->freepage?
> 
> It's too late.  The optimisation must be cancelled before there's a chance
> that a new page can be allocated and attached to the pagecache - but
> ->freepage() is called after the folio has been removed.  Doing it in
> ->freepage() would allow ->readahead(), ->readpage() or ->write_begin() to
> jump in and start a new read (which gets skipped because the optimisation is
> still in play).

OK.  You suggested that releasepage was an acceptable place to call it.
How about we have AS_RELEASE_ALL (... or something ...) and then
page_has_private() becomes a bit more complicated ... to the point
where we should probably get rid of it (by embedding it into
filemap_release_folio():

+++ b/mm/filemap.c
@@ -3981,6 +3981,9 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
        struct address_space * const mapping = folio->mapping;

        BUG_ON(!folio_test_locked(folio));
+       if (!mapping_needs_release(mapping) && !folio_test_private(folio) &&
+           !folio_test_private_2(folio))
+               return false;
        if (folio_test_writeback(folio))
                return false;