On Wed, May 25, 2022 at 11:10:41PM +1200, Barry Song wrote:

On Wed, May 25, 2022 at 7:14 AM Catalin Marinas [off-list ref] wrote:

quoted

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);
}

quoted

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

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