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Mon, 30 Mar 2026 15:09:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774883382; bh=WTEs+wWGfoGp7m+1pMCQulX7OTGdD9wt7oEgo7X/NC4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=czqt+VGBQH53ovNn2cfF9ycf8x3bSlQ8hpcOwAKfQ4OG7BYzHGkpsx2uai6eAvln2 bCpK6pfDZWoXyOquiOplKpu0D2kJq3+0SGS9puSXKpbdhlSYtZ+f4hcljD4WRsKhQI WlmBD+2zR9UDMPHKrgAS87tDd7pdmO+CzfdiBVcx7oIGSu8h30J1XGVuUmiq9hkN6Z 54dktGBKjl1oSvOF3EsqWS3zwqZ//hkCUliTuKxMEjMuJdzcbA3+erE+LDHtsSyi6s M7dSp0+dcBT4hJLbCBVLD/k2UZ+2GukCUGcms+6oBJhZlN1BryHvwhTdh2EabfGstX QzbM7F4K3m57A== Message-ID: Date: Mon, 30 Mar 2026 17:09:35 +0200 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v3 05/24] mm: thp: handle split failure in zap_pmd_range() To: Kiryl Shutsemau , Usama Arif Cc: Andrew Morton , Lorenzo Stoakes , willy@infradead.org, linux-mm@kvack.org, fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, maddy@linux.ibm.com, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, linux-s390@vger.kernel.org References: <20260327021403.214713-1-usama.arif@linux.dev> <20260327021403.214713-6-usama.arif@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/30/26 16:13, Kiryl Shutsemau wrote: > On Thu, Mar 26, 2026 at 07:08:47PM -0700, Usama Arif wrote: >> zap_pmd_range() splits a huge PMD when the zap range doesn't cover the >> full PMD (partial unmap). If the split fails, the PMD stays huge. >> Falling through to zap_pte_range() would dereference the huge PMD entry >> as a PTE page table pointer. >> >> Skip the range covered by the PMD on split failure instead. > > Ughh... This is hacky as hell. > >> The skip is safe across all call paths into zap_pmd_range(): >> >> - exit_mmap() and OOM reaper: the zap range covers entire VMAs, so >> every PMD is fully covered (next - addr == HPAGE_PMD_SIZE). The >> zap_huge_pmd() branch handles these without splitting. The split >> failure path is unreachable. >> >> - munmap / mmap overlay: vma_adjust_trans_huge() (called from >> __split_vma) splits any PMD straddling the VMA boundary before the >> VMA is split. If that PMD split fails, __split_vma() returns >> -ENOMEM and the munmap is aborted before reaching zap_pmd_range(). >> The split failure path is unreachable. >> >> - MADV_DONTNEED: advisory hint, the kernel is allowed to ignore it. >> The pages remain valid and accessible. A subsequent access returns >> existing data without faulting. > > Em, no. MADV_DONTNEED users expect memory to be zeroed after the > "advise" is complete. At very least you need to zero the skipped range. Fully agreed. This definitely needs more thought :) -- Cheers, David