linux-kernel - Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large folios

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Date:   Thu, 7 Dec 2023 11:03:58 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Ryan Roberts <ryan.roberts@....com>,
        Steven Price <steven.price@....com>, akpm@...ux-foundation.org,
        catalin.marinas@....com, will@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
        shy828301@...il.com, v-songbaohua@...o.com,
        wangkefeng.wang@...wei.com, willy@...radead.org, xiang@...nel.org,
        ying.huang@...el.com, yuzhao@...gle.com
Subject: Re: [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for
 large folios

>>
>>> not per-folio? I'm also not sure what it buys us - instead of reading a per-page
>>> flag we now have to read 128 bytes of tag for each page and check its zero.
>>
>> My point is, if that is the corner case, we might not care about that.
> 
> Hi David,

Hi!

> my understanding is that this is NOT a corner. Alternatively, it is
> really a common case.

If it happens with < 1% of all large folios on swapout/swapin, it's not 
the common case. Even if some scenarios you point out below can and will 
happen.

> 
> 1. a large folio can be partially unmapped when it is in swapche and
> after it is swapped out
> in all cases, its tags can be partially invalidated. I don't think
> this is a corner case, as long
> as userspaces are still working at the granularity of basepages, this
> is always going to
> happen. For example, userspace libc such as jemalloc can identify
> PAGESIZE, and use
> madvise(DONTNEED) to return memory to the kernel. Heap management is
> still working
> at the granularity of the basepage.
> 
> 2. mprotect on a part of a large folio as Steven pointed out.
> 
> 3.long term, we are working to swap-in large folios as a whole[1] just
> like swapping out large
> folios as a whole. for those ptes which are still contiguous swap
> entries, i mean, which
> are not unmapped by userspace after the large folios are swapped out
> to swap devices,
> we have a chance to swap in a whole large folio, we do have a chance
> to restore tags
> for the large folio without early-exit. but we still have a good
> chance to fall back to base
> page if we fail to allocate large folio, in this case, do_swap_page()
> still works at the
> granularity of basepage. and do_swap_page() will call swap_free(entry),  tags of
> 
> this particular page can be invalidated as a result.

I don't immediately see how that relates. You get a fresh small folio 
and simply load that tag from the internal datastructure. No messing 
with large folios required, because you don't have a large folio. So no 
considerations about large folio batch MTE tag restore apply.

> 
> 4. too many early-exit might be negative to performance.
> 
> 
> So I am thinking that in the future, we need two helpers,
> 1. void __arch_swap_restore(swp_entry_t entry, struct page *page);
> this is always needed to support page-level tag restore.
> 
> 2.  void arch_swap_restore(swp_entry_t entry, struct folio *folio);
> this can be a helper when we are able to swap in a whole folio. two
> conditions must be met
> (a). PTEs entries are still contiguous swap entries just as when large
> folios were swapped
> out.
> (b). we succeed in the allocation of a large folio in do_swap_page.
> 
> For this moment, we only need 1; we will add 2 in swap-in large folio series.
> 
> What do you think?

I agree that it's better to keep it simple for now.

-- 
Cheers,

David / dhildenb