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Howlett" , Suren Baghdasaryan , Matthew Wilcox , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 1/7] mm/mremap: introduce more mergeable mremap via MREMAP_RELOCATE_ANON Message-ID: <2e02e0ee-cee7-4096-a330-78b1b091bc5e@lucifer.local> References: <182bf1ce-1b67-4243-854b-4d0c26aae563@redhat.com> <21f89b73-aaae-4674-aea2-aefc7a4847d9@redhat.com> <9f81bfe4-4cc7-4754-b92f-db3a4e549f86@lucifer.local> <01803b0f-47ff-429c-a6af-33fee39b957b@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <01803b0f-47ff-429c-a6af-33fee39b957b@redhat.com> X-ClientProxiedBy: LO4P265CA0238.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:350::11) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|SA1PR10MB5710:EE_ X-MS-Office365-Filtering-Correlation-Id: a0c6436b-b281-4743-5c69-08dd705f083f 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wrote: > On 22.03.25 08:21, Lorenzo Stoakes wrote: > > On Sat, Mar 22, 2025 at 07:17:05AM +0100, David Hildenbrand wrote: > > > On 22.03.25 06:33, David Hildenbrand wrote: > > > > On 22.03.25 01:14, Jann Horn wrote: > > > > > On Fri, Mar 21, 2025 at 10:54 PM Lorenzo Stoakes > > > > > wrote: > > > > > > diff --git a/mm/mremap.c b/mm/mremap.c > > > > > > index 0865387531ed..bb67562a0114 100644 > > > > > > --- a/mm/mremap.c > > > > > > +++ b/mm/mremap.c > > > > > [...] > > > > > > +/* > > > > > > + * If the folio mapped at the specified pte entry can have its index and mapping > > > > > > + * relocated, then do so. > > > > > > + * > > > > > > + * Returns the number of pages we have traversed, or 0 if the operation failed. > > > > > > + */ > > > > > > +static unsigned long relocate_anon(struct pagetable_move_control *pmc, > > > > > > + unsigned long old_addr, unsigned long new_addr, pte_t pte, > > > > > > + bool undo) > > > > > > +{ > > > > > > + struct page *page; > > > > > > + struct folio *folio; > > > > > > + struct vm_area_struct *old, *new; > > > > > > + pgoff_t new_index; > > > > > > + unsigned long ret = 1; > > > > > > + > > > > > > + old = pmc->old; > > > > > > + new = pmc->new; > > > > > > + > > > > > > + /* Ensure we have truly got an anon folio. */ > > > > > > + page = vm_normal_page(old, old_addr, pte); > > > > > > + if (!page) > > > > > > + return ret; > > > > > > + folio = page_folio(page); > > > > > > + folio_lock(folio); > > > > > > + > > > > > > + /* no-op. */ > > > > > > + if (!folio_test_anon(folio) || folio_test_ksm(folio)) > > > > > > + goto out; > > > > > > + > > > > > > + /* > > > > > > + * This should not happen as we explicitly disallow this, but check > > > > > > + * anyway. > > > > > > + */ > > > > > > + if (folio_test_large(folio)) { > > > > > > + ret = 0; > > > > > > + goto out; > > > > > > + } > > > > > > > > > > Do I understand correctly that you assume here that the page is > > > > > exclusively mapped? Maybe we could at least > > > > > WARN_ON(folio_mapcount(folio) != 1) or something like that? > > > > > > > > > > (I was also wondering if the PageAnonExclusive bit is somehow > > > > > relevant, but we should probably not look at or touch that here, > > > > > unless we want to think about cases where we _used to_ have a child > > > > > from which the page may have been GUP'd...) > > > > > > > > UFFDIO_MOVE implements something similar. Right now we keep it simple: > > > > > > > > if (folio_test_large(src_folio) || > > > > folio_maybe_dma_pinned(src_folio) || > > > > !PageAnonExclusive(&src_folio->page)) { > > > > err = -EBUSY; > > > > goto out; > > > > } > > > > > > > > Whereby we > > > > > > > > a) Make sure we cover all PTEs (-> small folio, single PTE). Large > > > > PTE-mapped folios are split. > > > > > > > > b) Make sure there are no GUP pins (maybe not required here?) > > > > > > > > c) The folio is exclusive to this process > > > > > > On additional note as my memory comes back: if PAE is set, there cannot be > > > other (inactive) mappings from the swapcache. So whenever we use folio lock > > > + mapcount data, the possibility of the swapcache (having inactive mappings > > > from other processes etc.) must be considered. > > > > Ack, do you have a feel for how such a check would work? > > Likely under folio lock > > if (folio_test_swapcache(folio) && !folio_free_swap(folio)) { > /* unable to move. */ > folio_unlock(folio) > return -ENOTGOINGTOHAPPEN; > } Thanks! Will adapt. > > -- > Cheers, > > David / dhildenb >