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Message-ID: <6254ce2c-4a47-4501-b518-dedaddcbf91a@redhat.com> Date: Fri, 20 Dec 2024 20:06:48 +0100 From: David Hildenbrand <david@...hat.com> To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org, dan.j.williams@...el.com, linux-mm@...ck.org Cc: lina@...hilina.net, zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com, vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com, will@...nel.org, mpe@...erman.id.au, npiggin@...il.com, dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org, djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com, peterx@...hat.com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de, david@...morbit.com Subject: Re: [PATCH v4 12/25] mm/memory: Enhance insert_page_into_pte_locked() to create writable mappings On 20.12.24 20:01, David Hildenbrand wrote: > On 17.12.24 06:12, Alistair Popple wrote: >> In preparation for using insert_page() for DAX, enhance >> insert_page_into_pte_locked() to handle establishing writable >> mappings. Recall that DAX returns VM_FAULT_NOPAGE after installing a >> PTE which bypasses the typical set_pte_range() in finish_fault. >> >> Signed-off-by: Alistair Popple <apopple@...dia.com> >> Suggested-by: Dan Williams <dan.j.williams@...el.com> >> >> --- >> >> Changes since v2: >> >> - New patch split out from "mm/memory: Add dax_insert_pfn" >> --- >> mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++-------- >> 1 file changed, 37 insertions(+), 8 deletions(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 06bb29e..cd82952 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2126,19 +2126,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma, >> } >> >> static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte, >> - unsigned long addr, struct page *page, pgprot_t prot) >> + unsigned long addr, struct page *page, >> + pgprot_t prot, bool mkwrite) >> { >> struct folio *folio = page_folio(page); >> + pte_t entry = ptep_get(pte); >> pte_t pteval; >> >> - if (!pte_none(ptep_get(pte))) >> - return -EBUSY; >> + if (!pte_none(entry)) { >> + if (!mkwrite) >> + return -EBUSY; >> + >> + /* >> + * For read faults on private mappings the PFN passed in may not >> + * match the PFN we have mapped if the mapped PFN is a writeable >> + * COW page. In the mkwrite case we are creating a writable PTE >> + * for a shared mapping and we expect the PFNs to match. If they >> + * don't match, we are likely racing with block allocation and >> + * mapping invalidation so just skip the update. >> + */ > > Would it make sense to instead have here > > /* See insert_pfn(). */ > > But ... > >> + if (pte_pfn(entry) != page_to_pfn(page)) { >> + WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry))); >> + return -EFAULT; >> + } >> + entry = maybe_mkwrite(entry, vma); >> + entry = pte_mkyoung(entry); >> + if (ptep_set_access_flags(vma, addr, pte, entry, 1)) >> + update_mmu_cache(vma, addr, pte); > > ... I am not sure if we want the above at all. Someone inserted a page, > which is refcounted + mapcounted already. > > Now you ignore that and do like the second insertion "worked" ? > > No, that feels wrong, I suspect you will run into refcount+mapcount issues. > > If there is already something, inserting must fail IMHO. If you want to > change something to upgrade write permissions, then a different > interface should be used. Ah, now I realize that the early exit saves you because we won't adjust the refcount +mapcount. I still wonder if that really belongs in here, I would prefer to not play such tricks to upgrade write permissions if possible. -- Cheers, David / dhildenb
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