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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 88F98180003 X-Stat-Signature: 6c7nkzefj8stwc1trorrbjptycfhfu6f X-Rspam-User: X-HE-Tag: 1742621608-213084 X-HE-Meta: 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 kuIiXM2+ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 In general, things I can reason about with confidence are: a) As alternative to PageAnonExclusive(), we can check folio_mapcount()==1 under the folio lock for small folios / PMD-mapped folios. As you (Jann) say, there might be unexpected references on the folio from other processes. b) For large (pte-mapped) folios, we could try batching multiple PTEs (folio_pte_batch() etc.). We'd be processing all mappings with folio_lock + folio_mapcount() == #PTEs. c) In -next, there is now be the option to use folio lock + folio_maybe_mapped_shared() == false. But it doesn't tell you into how many VMAs a large folio is mapped into. In the following case: [ folio ] [ VMA#1 ] [ VMA#2 ] c) would not tell you if you are fine modifying the folio when moving VMA#2. -- Cheers, David / dhildenb