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Thu, 26 Jan 2023 09:29:10 -0800 (PST) MIME-Version: 1.0 References: <20230125233554.153109-1-surenb@google.com> <20230125233554.153109-5-surenb@google.com> <20230126151015.ru2m26jkhwib6x6u@techsingularity.net> <20230126172657.bmga5hy74ifsbhun@techsingularity.net> In-Reply-To: <20230126172657.bmga5hy74ifsbhun@techsingularity.net> From: Suren Baghdasaryan Date: Thu, 26 Jan 2023 09:28:59 -0800 Message-ID: Subject: Re: [PATCH v3 4/7] mm: replace vma->vm_flags direct modifications with modifier calls To: Mel Gorman Cc: akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 26, 2023 at 9:27 AM Mel Gorman wrote: > > On Thu, Jan 26, 2023 at 08:10:26AM -0800, Suren Baghdasaryan wrote: > > On Thu, Jan 26, 2023 at 7:10 AM Mel Gorman wrote: > > > > > > On Wed, Jan 25, 2023 at 03:35:51PM -0800, Suren Baghdasaryan wrote: > > > > Replace direct modifications to vma->vm_flags with calls to modifier > > > > functions to be able to track flag changes and to keep vma locking > > > > correctness. > > > > > > > > Signed-off-by: Suren Baghdasaryan > > > > Acked-by: Michal Hocko > > > > > > Acked-by: Mel Gorman > > > > > > Minor comments that are safe to ignore. > > > > > > I think a better name for mod_vm_flags is set_clear_vm_flags to hint that > > > the first flags are to be set and the second flags are to be cleared. > > > For this patch, it doesn't matter, but it might avoid accidental swapping > > > in the future. > > > > > > reset_vm_flags might also be better named as reinit_vma_flags (or > > > vma_flags_reinit). Maybe also encourage the use of [set|clear_mod]_vm_flags > > > where possible in the comment to track exactly what is changing and > > > why. Some cases like userfaultfd just want to clear __VM_UFFD_FLAGS but > > > altering the flow in this patch is inappropriate and error prone. Others > > > such as the infiniband changes and madvise are a lot more complex. > > > > That's a good point, but I don't want people to use mod_vm_flags() for > > the cases when the order of set/clear really matters. In such cases > > set_vm_flags() and clear_vm_flags() should be explicitly used. Maybe > > to make that clear I should add a comment and rewrite the functions > > as: > > > > void mod_vm_flags(vma, set, clear) { > > vma.vm_flags = vma.vm_flags | set & clear; > > } > > > > Offhand, I'm not thinking of a case where that really matters and as they > are not necessarily ordered, it's raising a read flag so yes, it definitely > it needs a comment if the ordering matters. > > > In this patchset it's not that obvious but mod_vm_flags() was really > > introduced in the original per-VMA lock patchset for efficiency to > > avoid taking extra per-VMA locks. A combo of > > set_vm_flags()+clear_vm_flags() would try to retake the same per-VMA > > lock in the second call while mod_vm_flags() takes the lock only once > > and does both operations. > > Ok, that seems fair but still needs a comment on why a mod_vm_flags is > not necessarily equivalent to a set_vm_flags + clear_vm_flags in terms of > correctness if that is indeed the case. > > > Not a huge overhead because we check if the > > lock is already taken and bail out early but still... > > So, would the above modification to mod_vm_flags() address your concern? > > > > My concerns are entirely with the callers, not the implementation. If > someone is modifying a call site using mod_vm_flags, they have to read > through all the preceding logic to ensure the final combination of flags > is valid. It's a code maintenance issue, not a correctness issue. Got it. I'll modify the implementation to make a single assignment and will add a comment to use only when order doesn't matter. Thanks! > > -- > Mel Gorman > SUSE Labs