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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qljeg-000g22-PN; Thu, 28 Sep 2023 05:33:43 +0000 Date: Thu, 28 Sep 2023 06:33:42 +0100 From: Matthew Wilcox To: Suren Baghdasaryan Cc: Andrew Morton , linux-mm@kvack.org Subject: Re: [PATCH 3/6] mm: Handle shared faults under the VMA lock Message-ID: References: <20230927052505.2855872-1-willy@infradead.org> <20230927052505.2855872-4-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Queue-Id: 650961A000E X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: xnintms6rgzxyy7sgu9aj4gjb7casb8n X-HE-Tag: 1695879226-391350 X-HE-Meta: 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 PxeR+2mf AxWZfEZYu+qfwZq2rVeboOXzzpk3nGabpDSgqM9h5xGNS7r0kuzfFHLamW+h7jpWpZG6KsmINLuI2jV+GsjnFK0rMRyLtgW4GQ9u56tpep+tScL7P09KH4ZrMy4YcE6m53U+ErmwdBP/015emjBO+47GSsuE8R8bPxL2JhW+AYXhyS1axpAUwhC4PwbnLb3YA7a/6uQH9BrqgdKgCeiz0aKz6wg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Sep 27, 2023 at 06:02:47PM -0700, Suren Baghdasaryan wrote: > On Wed, Sep 27, 2023 at 5:46 PM Suren Baghdasaryan wrote: > > > > On Tue, Sep 26, 2023 at 10:25 PM Matthew Wilcox (Oracle) > > wrote: > > > > > > There are many implementations of ->fault and some of them depend on > > > mmap_lock being held. All vm_ops that implement ->map_pages() end > > > up calling filemap_fault(), which I have audited to be sure it does > > > not rely on mmap_lock. So (for now) key off ->map_pages existing > > > as being a flag to indicate that it's safe to call ->fault while > > > only holding the vma lock. > > > > > > Signed-off-by: Matthew Wilcox (Oracle) > > > --- > > > mm/memory.c | 22 ++++++++++++++++++---- > > > 1 file changed, 18 insertions(+), 4 deletions(-) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index cff78c496728..0f3da4889230 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf) > > > count_vm_event(PGREUSE); > > > } > > > > > > +/* > > > + * We could add a bitflag somewhere, but for now, we know that all > > > + * vm_ops that have a ->map_pages have been audited and don't need > > > + * the mmap_lock to be held. > > > + */ > > > +static inline vm_fault_t vmf_maybe_unlock_vma(const struct vm_fault *vmf) > > > +{ > > > + struct vm_area_struct *vma = vmf->vma; > > > + > > > + if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK)) > > > + return 0; > > > + vma_end_read(vma); > > > + return VM_FAULT_RETRY; > > > +} > > > + > > > static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf) > > > { > > > struct vm_area_struct *vma = vmf->vma; > > > @@ -4669,10 +4684,9 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) > > > vm_fault_t ret, tmp; > > > struct folio *folio; > > > > > > - if (vmf->flags & FAULT_FLAG_VMA_LOCK) { > > > - vma_end_read(vma); > > > - return VM_FAULT_RETRY; > > > - } > > > + ret = vmf_maybe_unlock_vma(vmf); > > > > The name of this new function in this context does not seem > > appropriate to me. The logic of this check was that we can't rely on > > VMA lock since it might not be sufficient, so we have to retry with > > mmap_lock instead. With this change it seems like we intentionally try > > to unlock the VMA here. IMHO this would be more understandable: > > > > static inline bool is_vma_lock_sufficient(struct vm_area_struct *vma) { > > return vma->vm_ops->map_pages != NULL; > > } Originally I called this function vma_needs_mmap_lock() (with the opposite polarity). But I disliked the duplication of code ... > Same comment for the rest of the patches where vmf_maybe_unlock_vma() > is being used. It would be great to have this logic coded in one > function like you do but I could not find an appropriate name that > would convey that "we want to check if the current lock is sufficient > and if not then we will drop it and retry". Maybe you or someone else > can think of a good name for it? Maybe +static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf) +{ + struct vm_area_struct *vma = vmf->vma; + + if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK)) + return 0; + vma_end_read(vma); + return VM_FAULT_RETRY; +} I'm having trouble coming up with a name that doesn't imply it's a bool predicate.