Re: [PATCH 04/11] mm: add PG_dropbehind folio flag

[Matthew Wilcox]

On Fri, Dec 20, 2024 at 05:11:52PM +0200, Kirill A. Shutemov wrote:
> On Fri, Dec 20, 2024 at 02:38:09PM +0000, Matthew Wilcox wrote:
> > On Fri, Dec 20, 2024 at 01:08:39PM +0200, Kirill A. Shutemov wrote:
> > > On Fri, Dec 13, 2024 at 08:55:18AM -0700, Jens Axboe wrote:
> > > > Add a folio flag that file IO can use to indicate that the cached IO
> > > > being done should be dropped from the page cache upon completion.
> > > > 
> > > > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > > 
> > > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > > 
> > > + David, Vlastimil.
> > > 
> > > I think we should consider converting existing folio_set_reclaim() /
> > > SetPageReclaim() users to the new flag. From a quick scan, all of them
> > > would benefit from dropping the page after writeback is complete instead
> > > of leaving the folio on the LRU.
> > 
> > Ooh, that would be nice.  Removes the overloading of PG_reclaim with
> > PG_readahead, right?
> 
> Yep.

Then ... maybe this series should just coopt the PG_reclaim flag for its
purposes?

Going through the users:

lru_deactivate_file()
---------------------

Called due to mapping_try_invalidate() failing to invalidate.
Absolutely, get rid of this folio as quickly as possible.

pageout()
---------

Again, we're trying to get rid of this folio.  This time due to memory
pressure / reaching the end of the LRU list.  Yup, we want it gone.

shrink_folio_list()
-------------------

Again, end of the LRU (both cases in this function)

zswap_writeback_entry()
-----------------------

This is exactly the same case that Jens is adding.  The swapcache is
being used as a staging location, and we want the folio gone as soon as
writeback completes.