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

[Usama Arif]

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.


[1] 
https://lore.kernel.org/all/nes73bwc5p6yhwt5tw3upxcqrn5kenn6lvqb6exrf4yppmz6jx@ywhuevpkxlvh/

[2] 
https://lore.kernel.org/all/20240530102126.357438-1-usamaarif642@xxxxxxxxx/

[3] https://lore.kernel.org/all/20240530122715.GB1222079@xxxxxxxxxxx/

> > (b) do you properly handle if the page is pinned (or just got pinned)
> >       and we must not discard it?
>
> Oh, and I forgot, are you handling userfaultd as expected? IIRC there 
> are some really nasty side-effects with userfaultfd even when 
> userfaultfd is currently not registered for a VMA [1].
>
> [1] 
> https://lore.kernel.org/linux-mm/3a4b1027-df6e-31b8-b0de-ff202828228d@xxxxxxxxxx/
>
> What should work is replacing all-zero anonymous pages by the shared 
> zeropage iff the anonymous page is not pinned and we synchronize 
> against GUP fast. Well, and we handle possible concurrent writes 
> accordingly.
>
> KSM does essentially that when told to de-duplicate the shared 
> zeropage, and I was thinking a while ago if we would want a 
> zeropage-only KSM version that doesn't need stable tress and all that, 
> but only deduplicates zero-filled pages into the shared zeropage in a 
> safe way.
Thanks for the pointer to KSM code.