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b=RyCNiVmG0gwVoJRRdkUXv8gX6GNUVxZGm/FHUWdTHa0q0orLghemwOWIP47eGmlHO f+VIjcdFvrgMuOGvEK8W6gKmVp8wXZHb4AI4icfn5DbCTSE8Fh4QPZgOWdVWm6k+IH 1VRf0nXYJNWfi4yctD/yfryMwJURmBiBgG3cQUxOEJASeT4q4JvyZGBOgUp/h0S+f1 D3jfaAehdGBFcnVXliyRb0AZ/1Vw1kbG39s/eKe/RvADFHSvrr/v2l8lnmLe1ocpSr 9jwBy2HBn7esZ8m+jhevy9+2T1RNKBcTL+3g/CfJdvdFLl2CwozxkwB/P4xaQNOwlE VBbr66FrXu6Ng== Message-ID: <8e643f51-a47f-428e-9443-678326288fa6@kernel.org> Date: Wed, 9 Sep 2026 21:16:23 +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> 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: rspam02 X-Rspamd-Queue-Id: DA3C340008 X-Stat-Signature: bk4fnb4q4ch4sbky378y5s3j4u485e9c X-Rspam-User: X-HE-Tag: 1788981390-891565 X-HE-Meta: U2FsdGVkX19bW1XTgMEYU1VAA0zZ9iZJhwcJ67IJPGCjBv+H+Dt9nIn54GucH02BhRRDOGA5Sx9VoZAIEOaGwegTHug5afweftMbCDEE2jY1Mua7+cOSIO0CYQVLNlJejgcGLw18w5KobpZfDOYgLIdaM+44eVwkis6Hs0wzgP8hXITFmWpbYu7XiGgFqJoHrY7aKXOrRVLZeV4ebgNpVRhbW0pCEKkiCciHcnt9peoPjlpzFp9wXA5vwv+kIPI5hhUYFCAcD0TGbqOB+WwgweZJp9GgFdeSzfpxwOjVKcLoQN6eFCHRouZ13j/LB0mmOhCrLaDv09I6kq+yVul6f1ev2VJrfC9HsyRNuriD0ia7GlpkE8byZ5/Qguck+4pXYGS5rYQ7rbbsTo8GuTs6HjGcjIdL75s/JDu9aKN3Sq7jdE4GLzgf6MpfU2XKcZzb1YySbKT0nG4j8AKSwe6PH43lkRy69OPPZtaiIl6+18i7QXvmTeKfcJ+1uYcWSn7190B8+qjIHJPCel4uvfpQ21ENb16NE7QAT5OEFcDz3Zi7PCKIUQvx0TPM0d5dGdl+IwTO10cw/L5PX9fMN61m70myvnppDle5ChyhZUL+bZRYAuAWzBzp5Qp7wx4dmeOE+lCL26JHGSsrBQMr/DbR2XTG7rz2AmSu/mQdQbybleSp/mOU7AdSDzj8A+Eguc7QHQj1Vq2+JULH/G7K6F40rNxgva1ICxdpTfWlt1WUtVz/nKHW1sRpxEdrzQV85o0ctYVm6GlzLMDCKHRE8DnexBbONMGtJC9ttcSNzDxtDzZY1wVxx7LXyYiVOoUn7gZmjtFq6jnP9OrElYUmJFzdcxIX4F2Mjtjt+CIRWQhym04eprKz+ClUDvY36L19r20nZyDgM2uQ39wqSvwMmYclMN4aU3QOK/ezN2xISxi2taniDpVJ7hdV5wkwnkJIxOrBgLJHSmIo2xLhandCtSZ WJvReic5 UclKT5N7L7anDUd/y+VCF6OLfS9BpZurmdhOfv2YZ7Tqy49Qw+4devDLiLUavYnY3p2JaMq/NY18IK8hakUNQ9R3aQs+pudQ8Ic13Vz5NvVN4xe8117EHsE3Q+EnMhPGg48i2N/QLxicybVjC4GUZGNImM9tykpYfLP8UhUXX285GDDlrpZ3PIBtVVz/ZnW5WJN2O+rPyGosQgMrp7g/qL1RdFy9fiZ64HjDkFUbbKyut3LCI/8UqQAW25fScNQwo024gQI8pRKOZDBZDlI+27gpnmU8s0wTelHelDHihIA6Iy/iucR2/3wz0Xjq97a5jW6mlf3Sk0ZwB10Kj8cN1EwVi60kMWI13b4P3cUgFzMIkYVdAZmKnTznE3cYv+08zAlsM Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/9/26 20:28, Suren Baghdasaryan wrote: > On Wed, Sep 9, 2026 at 10:16 AM David Hildenbrand (Arm) > wrote: >> >> On 9/7/26 08:39, Suren Baghdasaryan wrote: >>> smap_gather_stats() interprets its start parameter to mean vma->vm_start >>> when it's set to 0. Eliminate this special interpretation and pass >>> vma->vm_start explicitly when needed. >>> >>> Since smap_gather_stats() operates within a single VMA, we can replace >>> walk_page_vma()/walk_page_range() calls with walk_page_range_vma() >>> which is simpler and also can be called while holding per-VMA lock. >>> >>> No functional change intended. >>> >>> Suggested by: Lorenzo Stoakes >>> Signed-off-by: Suren Baghdasaryan >>> --- >>> fs/proc/task_mmu.c | 40 ++++++++++++++++++++++------------------ >>> 1 file changed, 22 insertions(+), 18 deletions(-) >>> >>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >>> index 9908ba32f180..3351decd1172 100644 >>> --- a/fs/proc/task_mmu.c >>> +++ b/fs/proc/task_mmu.c >>> @@ -1246,20 +1246,27 @@ get_smaps_shmem_walk_ops(struct proc_maps_private *priv) >>> return &smaps_shmem_walk_vma_lock_ops; >>> } >>> >>> -/* >>> - * Gather mem stats from @vma with the indicated beginning >>> - * address @start, and keep them in @mss. >>> +/** >>> + * smap_gather_stats() - Gather mem stats from @vma. >>> + * @priv: proc maps private state. >>> + * @vma: The VMA to gather stats for. >>> + * @mss: The accumulated stats. >>> + * @start: The address from which to start. >>> * >>> - * Use vm_start of @vma as the beginning address if @start is 0. >>> + * This gathers stats for the whole of the VMA unless the lock was dropped >>> + * and VMA grew or got merged and we found it again, in which case we only >>> + * gather stats for the remainder of the VMA range. >>> */ >>> static void smap_gather_stats(struct proc_maps_private *priv, >>> struct vm_area_struct *vma, >>> - struct mem_size_stats *mss, unsigned long start) >>> + struct mem_size_stats *mss, >>> + unsigned long start) >>> { >>> const struct mm_walk_ops *ops = get_smaps_walk_ops(priv); >>> + const bool is_partial = start > vma->vm_start; >>> >>> /* Invalid start */ >>> - if (start >= vma->vm_end) >>> + if (start < vma->vm_start || start >= vma->vm_end) >>> return; >>> >>> if (vma == get_gate_vma(priv->lock_ctx.mm)) >>> @@ -1279,20 +1286,17 @@ static void smap_gather_stats(struct proc_maps_private *priv, >>> * Unless we know that the shmem object (or the part mapped by >>> * our VMA) has no swapped out pages at all. >>> */ >>> - unsigned long shmem_swapped = shmem_swap_usage(vma); >>> + const unsigned long shmem_swapped = shmem_swap_usage(vma); >>> + const bool shared_or_ro = vma_test(vma, VMA_SHARED_BIT) || >>> + !vma_test(vma, VMA_WRITE_BIT); >>> >>> - if (!start && (!shmem_swapped || (vma->vm_flags & VM_SHARED) || >>> - !(vma->vm_flags & VM_WRITE))) { >>> + if (!is_partial && (!shmem_swapped || shared_or_ro)) >>> mss->swap += shmem_swapped; >>> - } else { >>> + else >>> ops = get_smaps_shmem_walk_ops(priv); >>> - } >> >> Horrible, horrible code, really. But not your fault :) >> >> I think we can just make the shared_or_ro less odd by just checking for cow >> mappings (as described in the comment). >> >> const bool is_cow = vma_is_cow_mapping(vma); >> >> ... >> >> if (is_partial || (shmem_swapped && is_cow)) >> ops = get_smaps_shmem_walk_ops(priv); >> else >> mss->swap += shmem_swapped; >> >> That's almost in a form that I could understand what's happening. > > 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() > > 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 (in sanitize_fault_flags() we even disallow write faults entirely if VM_MAYWRITE is missing) 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 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 (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. + * * Unless we know that the shmem object (or the part mapped by * our VMA) has no swapped out pages at all. */ -- Cheers, David