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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772100386; bh=45Zns+iP/oDv+AJi9pnKB/hFBYoQKB7TJG74cHZZyrc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RVfx1aLC/rgzPLjR+YcRHQ9QbMkTt7VfNKPsmzbBDUyihBicyXDn+/CeSp0q5fh/F kjKWSDb2zclM9wXTPMaYjY7TuXRdjo3wVHYkEPQHl5TO3YrcmhTI8ElSfoLYFDjLno Tnpicv8R/PgEavrSqwmG9XtEDSkeupZDbiLG8GqGN4c3SVz6DRQDzMYlmr38dgumMv 4bGZ6u2W69bswr2nr6Vsj063bCaR5yAK150vHGgOtsLE9QwvWy6yRGXCivwyIyoFqk bv/Ix9i6Xr2zxWM0B04MCptDQmEspkcDA811RyRki4QGP+naiU+Jj+62wb86gvZBMx PceWJ5vHKtGJQ== Message-ID: Date: Thu, 26 Feb 2026 11:06:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/rmap: fix incorrect pte restoration for lazyfree folios To: Lance Yang Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, baohua@kernel.org, dev.jain@arm.com, harry.yoo@oracle.com, jannh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, riel@surriel.com, stable@kernel.org, vbabka@kernel.org References: <36e676b4-dc6f-45f7-b885-8685227ac6a8@kernel.org> <20260226070940.96226-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: okkp19m54oz1xm81kj4k81kkshwjuz46 X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5ECC9100002 X-HE-Tag: 1772100387-441561 X-HE-Meta: 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 uDk+IZ3b XJrrHxrcs8M2I+wUffHA5fQlYYXjLUTHCcIVqTVRhvETegm0KfORZFCpNnNOhQpzYrWzdKuYPe/jNfoKe7lb2PDPHLMn/UDw6eiPYut374MWtFBmLIP3CFrw6L3Zei8GOKQ4yl/TzzdU6t/o7mK57fFZ9eS6RURvt4YsocZYQVEXTi9Zzy9YGJF9ZhQJnAu2GJa/G1AUwe00aW8+ufY0Umn3HFF707DKxgATwYq5WGbLdS3GgxSsUR4d4mp4GYtZ59PXl Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/26/26 08:09, Lance Yang wrote: > > On Tue, Feb 24, 2026 at 05:01:50PM +0100, David Hildenbrand (Arm) wrote: >> On 2/24/26 12:43, Lorenzo Stoakes wrote: >>> >>> Sorry I misread the original mail rushing through this is old... so this is less >>> pressing than I thought (for some reason I thought it was merged last cycle...!) >>> but it's a good example of how stuff can go unnoticed for a while. >>> >>> In that case maybe a revert is a bit much and we just want the simplest possible >>> fix for backporting. >> >> Dev volunteered to un-messify some of the stuff here. In particular, to >> extend batching to all cases, not just some hand-selected ones. >> >> Support for file folios is on the way. >> >>> >>> But is the proposed 'just assume wrprotect' sensible? David? >> >> In general, I think so. If PTEs were writable, they certainly have >> PAE set. The write-fault handler can fully recover from that (as PAE is >> set). If it's ever a performance problem (doubt), we can revisit. >> >> I'm wondering whether we should just perform the wrprotect earlier: >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 0f00570d1b9e..19b875ee3fad 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -2150,6 +2150,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >> >> /* Nuke the page table entry. */ >> pteval = get_and_clear_ptes(mm, address, pvmw.pte, nr_pages); >> + >> + /* >> + * Our batch might include writable and read-only >> + * PTEs. When we have to restore the mapping, just >> + * assume read-only to not accidentally upgrade >> + * write permissions for PTEs that must not be >> + * writable. >> + */ >> + pteval = pte_wrprotect(pteval); >> + >> /* >> * We clear the PTE but do not flush so potentially >> * a remote CPU could still be writing to the folio >> >> >> Given that nobody asks for writability (pte_write()) later. >> >> Or does someone care? >> >> Staring at set_tlb_ubc_flush_pending()->pte_accessible() I am >> not 100% sure. Could pte_wrprotect() turn a PTE inaccessible on some >> architecture (write-only)? I don't think so. >> >> >> We have the following options: >> >> 1) pte_wrprotect(): fake that all was read-only. >> >> Either we do it like Dev suggests, or we do it as above early. >> >> The downside is that any code that might later want to know "was >> this possibly writable" would get that information. Well, it wouldn't >> get that information reliably *today* already (and that sounds a bit shaky). > > Makes sense to me :) > >> 2) Tell batching logic to honor pte_write() >> >> Sounds suboptimal for some cases that really don't care in the future. >> >> 3) Tell batching logic to tell us if any pte was writable: FPB_MERGE_WRITE >> >> ... then we know for sure whether any PTE was writable and we could >> >> (a) Pass it as we did before around to all checks, like pte_accessible(). >> >> (b) Have an explicit restore PTE where we play save. >> >> >> I raised to Dev in private that softdirty handling is also shaky, as we >> batch over that. Meaning that we could lose or gain softdirty PTE bits in >> a batch. > > I guess we won't lose soft_dirty bits - only gain them (false positive): > > 1) get_and_clear_ptes() merges dirty bits from all PTEs via pte_mkdirty() > 2) pte_mkdirty() atomically sets both _PAGE_DIRTY and _PAGE_SOFT_DIRTY on > all architectures that support soft_dirty (x86, s390, powerpc, riscv) > 3) set_ptes() uses pte_advance_pfn() which keeps all flags intact > > So if any PTE in the batch was dirty, all PTEs become soft_dirty after > restore. PTEs can be softdirty without being dirty. That over-complicates the situation. -- Cheers, David