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[2003:cb:c702:2e00:b9ea:114c:a3f5:327e]) by smtp.gmail.com with ESMTPSA id y1-20020a1c4b01000000b003d208eb17ecsm1328703wma.26.2022.12.09.02.24.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Dec 2022 02:24:56 -0800 (PST) Message-ID: <72a62bf5-7633-f35f-75fd-0222fd0ab49f@redhat.com> Date: Fri, 9 Dec 2022 11:24:55 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare Content-Language: en-US To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Muchun Song , John Hubbard , Andrea Arcangeli , James Houghton , Jann Horn , Rik van Riel , Miaohe Lin , Andrew Morton , Mike Kravetz , Nadav Amit References: <20221207203034.650899-1-peterx@redhat.com> <20221207203034.650899-9-peterx@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08.12.22 21:47, Peter Xu wrote: > On Thu, Dec 08, 2022 at 02:14:42PM +0100, David Hildenbrand wrote: >> On 07.12.22 21:30, Peter Xu wrote: >>> Since walk_hugetlb_range() walks the pgtable, it needs the vma lock >>> to make sure the pgtable page will not be freed concurrently. >>> >>> Reviewed-by: Mike Kravetz >>> Signed-off-by: Peter Xu >>> --- >>> arch/s390/mm/gmap.c | 2 ++ >>> fs/proc/task_mmu.c | 2 ++ >>> include/linux/pagewalk.h | 11 ++++++++++- >>> mm/hmm.c | 15 ++++++++++++++- >>> mm/pagewalk.c | 2 ++ >>> 5 files changed, 30 insertions(+), 2 deletions(-) >>> >>> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c >>> index 8947451ae021..292a54c490d4 100644 >>> --- a/arch/s390/mm/gmap.c >>> +++ b/arch/s390/mm/gmap.c >>> @@ -2643,7 +2643,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr, >>> end = start + HPAGE_SIZE - 1; >>> __storage_key_init_range(start, end); >>> set_bit(PG_arch_1, &page->flags); >>> + hugetlb_vma_unlock_read(walk->vma); >>> cond_resched(); >>> + hugetlb_vma_lock_read(walk->vma); >>> return 0; >>> } >>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >>> index e35a0398db63..cf3887fb2905 100644 >>> --- a/fs/proc/task_mmu.c >>> +++ b/fs/proc/task_mmu.c >>> @@ -1613,7 +1613,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask, >>> frame++; >>> } >>> + hugetlb_vma_unlock_read(walk->vma); >>> cond_resched(); >>> + hugetlb_vma_lock_read(walk->vma); >> >> We already hold the mmap_lock and reschedule. Even without the >> cond_resched() we might happily reschedule on a preemptive kernel. So I'm >> not sure if this optimization is strictly required or even helpful in >> practice here. > > It's just low hanging fruit if we need that complexity anyway. > > That's also why I didn't do that for v1 (where I missed hmm special case, > though..), but I think since we'll need that anyway, we'd better release > the vma lock if we can easily do so. > > mmap_lock is just more special because it needs more work in the caller to > release (e.g. vma invalidations). Otherwise I'm happy dropping that too. > >> >> In the worst case, concurrent unsharing would have to wait. >> For example, s390_enable_skey() is called at most once for a VM, for most >> VMs it gets never even called. >> >> Or am I missing something important? > > Nothing important. I just don't see why we need to strictly follow the > same release rule of mmap_lock here when talking about vma lock. > > In short - if we can drop a lock earlier before sleep, why not? > > I tend to just keep it as-is, but let me know if you have further thoughts > or concerns. To me this looks like a possibly unnecessary optimization, requiring additional code. IMHO, possibly unnecessary unlock+relock makes thatthat code harder to get. For such cases, it would be good to have any evidence that it really helps. -- Thanks, David / dhildenb