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Message-ID: <bf9f79df-4821-4053-a1e9-f054f2ca5734@bytedance.com>
Date: Tue, 6 Aug 2024 11:31:24 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: david@...hat.com, hughd@...gle.com, willy@...radead.org, mgorman@...e.de,
muchun.song@...ux.dev, vbabka@...nel.org, akpm@...ux-foundation.org,
zokeefe@...gle.com, rientjes@...gle.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC PATCH v2 0/7] synchronously scan and reclaim empty user PTE
pages
Hi all,
On 2024/8/5 20:55, Qi Zheng wrote:
[...]
>
> 2. When we use mmu_gather to batch flush tlb and free PTE pages, the TLB is not
> flushed before pmd lock is unlocked. This may result in the following two
> situations:
>
> 1) Userland can trigger page fault and fill a huge page, which will cause
> the existence of small size TLB and huge TLB for the same address.
>
> 2) Userland can also trigger page fault and fill a PTE page, which will
> cause the existence of two small size TLBs, but the PTE page they map
> are different.
>
> For case 1), according to Intel's TLB Application note (317080), some CPUs of
> x86 do not allow it:
>
> ```
> If software modifies the paging structures so that the page size used for a
> 4-KByte range of linear addresses changes, the TLBs may subsequently contain
> both ordinary and large-page translations for the address range.12 A reference
> to a linear address in the address range may use either translation. Which of
> the two translations is used may vary from one execution to another and the
> choice may be implementation-specific.
>
> Software wishing to prevent this uncertainty should not write to a paging-
> structure entry in a way that would change, for any linear address, both the
> page size and either the page frame or attributes. It can instead use the
> following algorithm: first mark the relevant paging-structure entry (e.g.,
> PDE) not present; then invalidate any translations for the affected linear
> addresses (see Section 5.2); and then modify the relevant paging-structure
> entry to mark it present and establish translation(s) for the new page size.
> ```
>
> We can also learn more information from the comments above pmdp_invalidate()
> in __split_huge_pmd_locked().
>
> For case 2), we can see from the comments above ptep_clear_flush() in
> wp_page_copy() that this situation is also not allowed. Even without
> this patch series, madvise(MADV_DONTNEED) can also cause this situation:
>
> CPU 0 CPU 1
>
> madvise (MADV_DONTNEED)
> --> clear pte entry
> pte_unmap_unlock
> touch and tlb miss
> --> set pte entry
> mmu_gather flush tlb
>
> But strangely, I didn't see any relevant fix code, maybe I missed something,
> or is this guaranteed by userland?
I'm still quite confused about this, is there anyone who is familiar
with this part?
Thanks,
Qi
>
> Anyway, this series defines the following two functions to be implemented by
> the architecture. If the architecture does not allow the above two situations,
> then define these two functions to flush the tlb before set_pmd_at().
>
> - arch_flush_tlb_before_set_huge_page
> - arch_flush_tlb_before_set_pte_page
>
[...]
>
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