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b=FVC83b50kI9fTgdOqjSt0kDMiYCj/7+sPjR1D4ltQlsM5JqgXGLHiqA+sTdxfUykW utGrVdpgSobAFwfjuGjPOa6a8rKOJBlYKACGKVb7ZvKG3ktxfJbmL6WTpUdLX9H9b4 /oBX76rNcPEsARP31BVj2gDbh2fYzgX2OB4VQMTUwjitUnWdO36LRb94iB2kWEwwJ8 1n8ysI1hdpKmjOkXc73kYthtoJ12Qaq/4U82hqEzXXFgumMGTeHlLokAKPRherE8Uk wP30lvOKLLiDtINvFwaDBGm9GzOJv6p1xJE9eJj85Zh5T59JVzMQqO4HjkFeCyg7mu SFAeUbpvmF0nw== Message-ID: Date: Mon, 3 Aug 2026 17:18:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFT 0/3] mm, drm: ensure .fault() does not have to be followed by .pfn_mkwrite() for write faults To: Paolo Bonzini , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Boris Brezillon , Thomas Zimmermann , Michal Hocko , Sergio Lopez , Christian Koenig , Huang Rui , bcm-kernel-feedback-list@broadcom.com, dri-devel@lists.freedesktop.org, linux-mm@kvack.org References: <20260731164341.1109827-1-pbonzini@redhat.com> <8d94897f-0f56-4ecf-bcc8-50bffd6750b2@kernel.org> <58fc5ce1-b6f5-48df-bd3b-054443c60cdf@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C9A3B20007 X-Stat-Signature: e79j7ydf9acdy4rarc4opystr5zwtzkg X-Rspam-User: X-HE-Tag: 1785770287-764431 X-HE-Meta: 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 MgS1BzIH RclWuFt9Q3an7A/E1YWSwYwe79yA7a5nS7P4VkkDkkThS7nybMMMlNMMow+QVL3C6vBnD1JOQGUtdH1/gPESM0BHws8DBT9DeEsevNjPQz4LPsLm7yZb93CMEvzvtWpdT34mdtFHvNuyVB2YP7AAavSFr6QGpa6KEEXAtn9VOZT7Fcw4AUpknCXTnZ0KXj4QUZK3bV3rWe65j3jx3+vGyxRFpjj1opUtgO64QuvClJPoLm051c2JlcIc2HE8P3o1ylD1pz5uxGuMvWJjD8s1A+JaZ+HsWU0w12jE04b1Dgj918yCs0SONqnWCacRMOIYPxpOYV5xErBTrrLc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/3/26 16:19, Paolo Bonzini wrote: > On 8/3/26 13:54, David Hildenbrand (Arm) wrote: >> On 7/31/26 18:43, Paolo Bonzini wrote: >>> Warning - DRM parts (i.e. most of the patches) untested; I have Cc'd >>> the reporter to help with testing these patches. >>> >>> Right now, users of .pfn_mkwrite() have no way to create a PTE >>> that has gone through maybe_mkwrite().  Because vma_set_page_prot() >>> will have cleared the writable PTE bit, users of fixup_user_fault() >>> will see a read-only PTE and have no clue that the page needs >>> a *second* fault to reach its final status. >>> >>> Handling this in fixup_user_fault() is problematic: the information >>> about the presence of *_mkwrite is only recorded in vma->vm_page_prot, >>> which is an opaque pgprot_t, therefore only follow_pfnmap_start() >>> knows how to retrieve it. >> >> How is mprotect() supposed to work in that case? > > Hi David, Hi! > > not sure what you are worried about specifically, but fixup_user_fault() catches !VM_WRITE VMAs and returns early (see vma_permits_fault()). That part is clear, I was wondering about the following: mprotect(PROT_READ) followed by mprotect(PROT_READ | PROT_WRITE) You'd similarly end up without the writable bit in the PTE, and apparently there is not really a way to recover from this. Maybe that's just ok (just sounded odd :) ). > > Also, do_wp_page() has the comment: > >         /* >          * Shared mapping: we are guaranteed to have VM_WRITE and >          * FAULT_FLAG_WRITE set at this point. >          */ > > before the call to wp_pfn_shared() which is where .pfn_mkwrite() is called. > > Let me know if this was not what you were asking. > > Thanks for the review of patch 1---I mentioned here in the cover letter that the name was temporary and I'll take your suggestion.  I can either use EXPORT_SYMBOL_GPL or switch to inlines, but not > both because the existing functions like vmf_insert_pfn_prot() need to stay non-GPL-only. > > Anyhow, now that the series has a Tested-by I'll clean up everything, and repost later this week. Thanks! -- Cheers, David