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@xxxxxxx> 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
