Re: [PATCH v2 1/2] mm: clear pte for folios that are zero filled

[David Hildenbrand]

On 07.06.24 12:24, Usama Arif wrote:
> On 04/06/2024 13:43, David Hildenbrand wrote:
> > On 04.06.24 14:30, David Hildenbrand wrote:
> > > On 04.06.24 12:58, Usama Arif wrote:
> > > > Approximately 10-20% of pages to be swapped out are zero pages [1].
> > > > Rather than reading/writing these pages to flash resulting
> > > > in increased I/O and flash wear, the pte can be cleared for those
> > > > addresses at unmap time while shrinking folio list. When this
> > > > causes a page fault, do_pte_missing will take care of this page.
> > > > With this patch, NVMe writes in Meta server fleet decreased
> > > > by almost 10% with conventional swap setup (zswap disabled).
> > > >
> > > > [1]
> > > > https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
> > > >
> > > > Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> > > > ---
> > > >     include/linux/rmap.h |   1 +
> > > >     mm/rmap.c            | 163
> > > > ++++++++++++++++++++++---------------------
> > > >     mm/vmscan.c          |  89 ++++++++++++++++-------
> > > >     3 files changed, 150 insertions(+), 103 deletions(-)
> > > >
> > > > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > > > index bb53e5920b88..b36db1e886e4 100644
> > > > --- a/include/linux/rmap.h
> > > > +++ b/include/linux/rmap.h
> > > > @@ -100,6 +100,7 @@ enum ttu_flags {
> > > >                          * do a final flush if necessary */
> > > >         TTU_RMAP_LOCKED        = 0x80,    /* do not grab rmap lock:
> > > >                          * caller holds it */
> > > > +    TTU_ZERO_FOLIO        = 0x100,/* zero folio */
> > > >     };
> > > >        #ifdef CONFIG_MMU
> > > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > > index 52357d79917c..d98f70876327 100644
> > > > --- a/mm/rmap.c
> > > > +++ b/mm/rmap.c
> > > > @@ -1819,96 +1819,101 @@ static bool try_to_unmap_one(struct folio
> > > > *folio, struct vm_area_struct *vma,
> > > >                  */
> > > >                 dec_mm_counter(mm, mm_counter(folio));
> > > >             } else if (folio_test_anon(folio)) {
> > > > -            swp_entry_t entry = page_swap_entry(subpage);
> > > > -            pte_t swp_pte;
> > > > -            /*
> > > > -             * Store the swap location in the pte.
> > > > -             * See handle_pte_fault() ...
> > > > -             */
> > > > -            if (unlikely(folio_test_swapbacked(folio) !=
> > > > -                    folio_test_swapcache(folio))) {
> > > > +            if (flags & TTU_ZERO_FOLIO) {
> > > > +                pte_clear(mm, address, pvmw.pte);
> > > > +                dec_mm_counter(mm, MM_ANONPAGES);
> > >
> > > Is there an easy way to reduce the code churn and highlight the added
> > > code?
> > >
> > > Like
> > >
> > > } else if (folio_test_anon(folio) && (flags & TTU_ZERO_FOLIO)) {
> > >
> > > } else if (folio_test_anon(folio)) {
> > >
> > >
> > >
> > > Also to concerns that I want to spell out:
> > >
> > > (a) what stops the page from getting modified in the meantime? The CPU
> > >        can write it until the TLB was flushed.
> Thanks for pointing this out David and Shakeel. This is a big issue in
> this v2, and as Shakeel pointed out in [1] we need to do a second rmap
> walk. Looking at how ksm deals with this in try_to_merge_one_page which
> calls write_protect_page for each vma (i.e. basically an rmap walk),
> this would be much more CPU expensive and complicated compared to v1
> [2], where the swap subsystem can handle all complexities. I will go
> back to my v1 solution for the next revision as its much more simpler
> and the memory usage is very low (0.003%) as pointed out by Johannes [3]
> which would likely go away with the memory savings of not having a
> zswap_entry for zero filled pages, and the solution being a lot simpler
> than what a valid v2 approach would look like.

Agreed.

--
Cheers,

David / dhildenb