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[2003:cb:c70d:fb00:d3ed:5f44:1b2d:12af]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-439872b5a46sm47439565e9.32.2025.02.18.03.37.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Feb 2025 03:37:31 -0800 (PST) Message-ID: Date: Tue, 18 Feb 2025 12:37:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 19/20] fs/dax: Properly refcount fs dax pages To: Alistair Popple , akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org Cc: 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 Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250218_033738_716434_6E3CC97A X-CRM114-Status: GOOD ( 36.35 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 18.02.25 04:55, Alistair Popple wrote: > Currently fs dax pages are considered free when the refcount drops to > one and their refcounts are not increased when mapped via PTEs or > decreased when unmapped. This requires special logic in mm paths to > detect that these pages should not be properly refcounted, and to > detect when the refcount drops to one instead of zero. > > On the other hand get_user_pages(), etc. will properly refcount fs dax > pages by taking a reference and dropping it when the page is > unpinned. > > Tracking this special behaviour requires extra PTE bits > (eg. pte_devmap) and introduces rules that are potentially confusing > and specific to FS DAX pages. To fix this, and to possibly allow > removal of the special PTE bits in future, convert the fs dax page > refcounts to be zero based and instead take a reference on the page > each time it is mapped as is currently the case for normal pages. > > This may also allow a future clean-up to remove the pgmap refcounting > that is currently done in mm/gup.c. > > Signed-off-by: Alistair Popple > Reviewed-by: Dan Williams A couple of nits (sorry that I didn't manage to review the whole thing the last time, I am a slow reviewer ...). Likely that can all be adjsuted on top, no need for a full resend IMHO. > index 6674540..cf96f3d 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -71,6 +71,11 @@ static unsigned long dax_to_pfn(void *entry) > return xa_to_value(entry) >> DAX_SHIFT; > } > > +static struct folio *dax_to_folio(void *entry) > +{ > + return page_folio(pfn_to_page(dax_to_pfn(entry))); Nit: return pfn_folio(dax_to_pfn(entry)); > +} > + [...] > > -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? > + if (ref) > + return ref; > + > + folio->mapping = NULL; > + order = folio_order(folio); > + if (!order) > + return 0; > + > + for (i = 0; i < (1UL << order); i++) { > + struct dev_pagemap *pgmap = page_pgmap(&folio->page); > + struct page *page = folio_page(folio, i); > + struct folio *new_folio = (struct folio *)page; > + > + ClearPageHead(page); > + clear_compound_head(page); > + > + new_folio->mapping = NULL; > + /* > + * Reset pgmap which was over-written by > + * prep_compound_page(). > + */ > + new_folio->pgmap = pgmap; > + new_folio->share = 0; > + WARN_ON_ONCE(folio_ref_count(new_folio)); > + } > + > + return ref; > +} > + > +static void dax_folio_init(void *entry) > +{ > + struct folio *folio = dax_to_folio(entry); > + int order = dax_entry_order(entry); > + > + /* > + * Folio should have been split back to order-0 pages in > + * dax_folio_put() when they were removed from their > + * final mapping. > + */ > + WARN_ON_ONCE(folio_order(folio)); > + > + if (order > 0) { > + prep_compound_page(&folio->page, order); > + if (order > 1) > + INIT_LIST_HEAD(&folio->_deferred_list); Nit: prep_compound_page() -> prep_compound_head() should be taking care of initializing all folio fields already, so this very likely can be dropped. > + WARN_ON_ONCE(folio_ref_count(folio)); > + } > } [...] > } > @@ -1808,7 +1843,8 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf, > loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT; > bool write = iter->flags & IOMAP_WRITE; > unsigned long entry_flags = pmd ? DAX_PMD : 0; > - int err = 0; > + struct folio *folio; > + int ret, err = 0; > pfn_t pfn; > void *kaddr; > > @@ -1840,17 +1876,19 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf, > return dax_fault_return(err); > } > > + folio = dax_to_folio(*entry); > if (dax_fault_is_synchronous(iter, vmf->vma)) > return dax_fault_synchronous_pfnp(pfnp, pfn); > > - /* insert PMD pfn */ > + folio_ref_inc(folio); Why is that not a folio_get() ? Could the refcount be 0? Might deserve a comment then. > if (pmd) > - return vmf_insert_pfn_pmd(vmf, pfn, write); > + ret = vmf_insert_folio_pmd(vmf, pfn_folio(pfn_t_to_pfn(pfn)), > + write); > + else > + ret = vmf_insert_page_mkwrite(vmf, pfn_t_to_page(pfn), write); > + folio_put(folio); > > - /* insert PTE pfn */ > - if (write) > - return vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn); > - return vmf_insert_mixed(vmf->vma, vmf->address, pfn); > + return ret; > } > > static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp, > @@ -2089,6 +2127,7 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order) > { > struct address_space *mapping = vmf->vma->vm_file->f_mapping; > XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, order); > + struct folio *folio; > void *entry; > vm_fault_t ret; > > @@ -2106,14 +2145,17 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order) > xas_set_mark(&xas, PAGECACHE_TAG_DIRTY); > dax_lock_entry(&xas, entry); > xas_unlock_irq(&xas); > + folio = pfn_folio(pfn_t_to_pfn(pfn)); > + folio_ref_inc(folio); Same thought. > 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()? 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. -- Cheers, David / dhildenb