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[2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43985c50397sm113532355e9.0.2025.02.20.03.51.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 03:51:43 -0800 (PST) Message-ID: Date: Thu, 20 Feb 2025 12:51:41 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 19/20] fs/dax: Properly refcount fs dax pages To: Alistair Popple Cc: akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org, Alison Schofield , lina@asahilina.net, zhang.lyra@gmail.com, gerald.schaefer@linux.ibm.com, vishal.l.verma@intel.com, dave.jiang@intel.com, logang@deltatee.com, bhelgaas@google.com, jack@suse.cz, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, npiggin@gmail.com, dave.hansen@linux.intel.com, ira.weiny@intel.com, willy@infradead.org, djwong@kernel.org, tytso@mit.edu, linmiaohe@huawei.com, peterx@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, david@fromorbit.com, chenhuacai@kernel.org, kernel@xen0n.name, loongarch@lists.linux.dev References: From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >>> -static inline unsigned long dax_folio_share_put(struct folio *folio) >>> +static inline unsigned long dax_folio_put(struct folio *folio) >>> { >>> - return --folio->page.share; >>> + unsigned long ref; >>> + int order, i; >>> + >>> + if (!dax_folio_is_shared(folio)) >>> + ref = 0; >>> + else >>> + ref = --folio->share; >>> + >> >> out of interest, what synchronizes access to folio->share? > > Actually that's an excellent question as I hadn't looked too closely at this > given I wasn't changing the overall flow with regards to synchronization, merely > representation of the "shared" state. So I don't have a good answer for you off > the top of my head - Dan maybe you can shed some light here? Not that I understand what that dax-shared thing is or does, but the non-atomic update on a folio_put path looked "surprising". >>> diff --git a/include/linux/dax.h b/include/linux/dax.h >>> index 2333c30..dcc9fcd 100644 >>> --- a/include/linux/dax.h >>> +++ b/include/linux/dax.h >>> @@ -209,7 +209,7 @@ int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero, >> >> [...] >> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index d189826..1a0d6a8 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -2225,7 +2225,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >>> tlb->fullmm); >>> arch_check_zapped_pmd(vma, orig_pmd); >>> tlb_remove_pmd_tlb_entry(tlb, pmd, addr); >>> - if (vma_is_special_huge(vma)) { >>> + if (!vma_is_dax(vma) && vma_is_special_huge(vma)) { >> >> I wonder if we actually want to remove the vma_is_dax() check from >> vma_is_special_huge(), and instead add it to the remaining callers of >> vma_is_special_huge() that still need it -- if any need it. >> >> Did we sanity-check which callers of vma_is_special_huge() still need it? Is >> there still reason to have that DAX check in vma_is_special_huge()? > > If by "we" you mean "me" then yes :) There are still a few callers of it, mainly > for page splitting. Heh, "you or any of the reviewers" :) So IIUC, the existing users still need the DAX check I assume (until that part is cleaned up, below). > >> But vma_is_special_huge() is rather confusing from me ... the whole >> vma_is_special_huge() thing should probably be removed. That's a cleanup for >> another day, though. > > But after double checking I have come to the same conclusion as you - it should > be removed. I will add that to my ever growing clean-up series that can go on > top of this one. Nice! -- Cheers, David / dhildenb