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b=hum0N2mvW/CuBfRXSaUF604iUi2nR0bxw6UgLkHyRZDJuNLy1Ha9yCWp1/5OdGC0t mHffGzrg+Jn8zhnmUzT8u1hPEPvI+GhxC/gW76xY6DMQN/2vp2JvZpFLP40DRKW9vN o1uTlOXTl3NlV4t+K4WwIwp/rwA044erTzIMl5LdME8z41WwXqm3XyZF/HdX8KjFVl 8pPMSbrivX76vlMVZBnVs9Odk7O2B/rH+q1saii4dYE2iCECTnOH5sQOFTP/R+tfhM PDggJLUBhRMQdtdq2kBlokqWj6Ooog/GtNWD0U81w6DYG0gy1yFXUCfOVWFzEkxup4 WsUe7aEXtvMpA== Message-ID: <2336140f-c993-467a-b2eb-e99dc9427a59@kernel.org> Date: Thu, 10 Sep 2026 09:41:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 3/5] proc/task_mmu: remove special-casing of smap_gather_stats() start parameter To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, willy@infradead.org, jannh@google.com, paulmck@kernel.org, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20260907063918.3432401-1-surenb@google.com> <20260907063918.3432401-4-surenb@google.com> <4825a2fe-f064-45d7-a7c9-e98de7efe86c@kernel.org> <8e643f51-a47f-428e-9443-678326288fa6@kernel.org> 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-Rspam-User: X-Rspamd-Queue-Id: BA0E4140005 X-Stat-Signature: g7qpww3p5nf5coos43nnzpqswjnoxak8 X-Rspamd-Server: rspam01 X-HE-Tag: 1789026115-767866 X-HE-Meta: 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 5m+7j9cm Al31lsDWR0wptpxiSQd+BzNcspD56ucAcnyi8G1SZnMJ82k30Qd2jaFN/SgHzfIwm7wmJncq2b+2J1S9YipRrIoXNGj12yOidiQzqXCXVqsny/Gep86ZjHjZsxj84OE0UoUwCX1OqUskskTG0QX1QE+dJZGk5BBwHVbk2LfB1rYMSziLx4wSKMdV1IuSlLcVynlrbOifpwI59L36NRFUPnfJvtBcZl5rmIdfkhYsa7ZdhOQAK88QJytA+mIdPne31SGS5HOQ5vhtwSROX1xHFzTYw9BTJSYprr8uiRH/pv77pkWEfruHF52Izc5LuabPYK3WPCKPNJqkqLZAU9c6JRjGgnj9wPlZWTQQF Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/9/26 23:51, Suren Baghdasaryan wrote: > On Wed, Sep 9, 2026 at 12:16 PM David Hildenbrand (Arm) > wrote: >> >> On 9/9/26 20:28, Suren Baghdasaryan wrote: >>> On Wed, Sep 9, 2026 at 10:16 AM David Hildenbrand (Arm) >>> wrote: >>> >>> Hmm. So, are you saying that !is_cow always implies shared_or_ro? Or >>> maybe you are stating that vma_is_cow_mapping() was the actual intent >>> here? >> >> So the comment says: >> >> "For private writable mappings, we might have COW pages that .." >> >> Which translates to: >> >> private writable == vma_is_cow_mapping() > > Ok, just want to make sure I'm not missing something subtle. > >> >>> >>> shared_or_ro = VMA_SHARED_BIT || !VMA_WRITE_BIT >>> >>> is_cow = !VMA_SHARED_BIT && VMA_MAYWRITE_BIT >>> !is_cow = VMA_SHARED_BIT || !VMA_MAYWRITE_BIT >>> >>> so, !is_cow would impy shared_or_ro only if !VMA_MAYWRITE_BIT always >>> implies !VMA_WRITE_BIT. But I think it's possible to have a VMA that >>> has VMA_WRITE_BIT but not VMA_MAYWRITE_BIT, right? >> >> VMA_WRITE should imply VMA_MAYWRITE > > Ah, good to know. > >> >> (in sanitize_fault_flags() we even disallow write faults entirely if VM_MAYWRITE >> is missing) > > I see. > >> >> For example, a >>> driver can create such a VMA to allow writing to the VMA but to lock >>> its content once mprotect(PROT_READ) gets called. >> >> I don't think that would be valid for a driver to do. But it wouldn't matter >> here because >> >> shmem_mapping(vma->vm_file->f_mapping) > > I was obviously overthinking this :) > >> >> >> I think we could simplify the comment as well to: >> >> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >> index e671b4fd8dedd..4b7e7089cafa7 100644 >> --- a/fs/proc/task_mmu.c >> +++ b/fs/proc/task_mmu.c >> @@ -1303,12 +1303,10 @@ static void smap_gather_stats(struct proc_maps_private >> *priv,if (is_partial || (shmem_swapped && is_cow)) >> >> if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) { >> /* >> - * For shared or readonly shmem mappings we know that all >> - * swapped out pages belong to the shmem object, and we can >> - * obtain the swap value much more efficiently. For private >> - * writable mappings, we might have COW pages that are >> - * not affected by the parent swapped out pages of the shmem >> - * object, so we have to distinguish them during the page walk. >> + * In CoW mappings, we might have anon folios that are >> + * independent of the shmem object. So fallback to the less >> + * efficient mechanism in such mappings. >> + * > > Sounds good. Will update. Likely worth putting that into a prior cleanup patch, so there is less noise in this patch. -- Cheers, David