Re: [PATCH] arm64: enable THP_SWAP for arm64

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, May 25, 2022 at 11:10:41PM +1200, Barry Song wrote:
> On Wed, May 25, 2022 at 7:14 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > I think this should work and with your other proposal it would be
> > limited to MTE pages:
> >
> > #define arch_thp_swp_supported(page)    (!test_bit(PG_mte_tagged, &page->flags))
> >
> > Are THP pages loaded from swap as a whole or are they split? IIRC the
> 
> i can confirm thp is written as a whole through:
> [   90.622863]  __swap_writepage+0xe8/0x580
> [   90.622881]  swap_writepage+0x44/0xf8
> [   90.622891]  pageout+0xe0/0x2a8
> [   90.622906]  shrink_page_list+0x9dc/0xde0
> [   90.622917]  shrink_inactive_list+0x1ec/0x3c8
> [   90.622928]  shrink_lruvec+0x3dc/0x628
> [   90.622939]  shrink_node+0x37c/0x6a0
> [   90.622950]  balance_pgdat+0x354/0x668
> [   90.622961]  kswapd+0x1e0/0x3c0
> [   90.622972]  kthread+0x110/0x120
> 
> but i have never got a backtrace in which thp is loaded as a whole though it
> seems the code has this path:
> int swap_readpage(struct page *page, bool synchronous)
> {
>         ...
>         bio = bio_alloc(sis->bdev, 1, REQ_OP_READ, GFP_KERNEL);
>         bio->bi_iter.bi_sector = swap_page_sector(page);
>         bio->bi_end_io = end_swap_bio_read;
>         bio_add_page(bio, page, thp_size(page), 0);
>         ...
>         submit_bio(bio);
> }
> 
> > splitting still happens but after the swapping out finishes. Even if
> > they are loaded as 4K pages, we still have the mte_save_tags() that only
> > understands small pages currently, so rejecting THP pages is probably
> > best.
> 
> as anyway i don't have a mte-hardware to do a valid test to go any
> further, so i will totally disable thp_swp for hardware having mte for
> this moment in patch v2.

It makes sense. If we decide to improve this for MTE, we'll change the
arch check.

Thanks.

-- 
Catalin