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David Hildenbrand authored
Staring at the comment "Recheck VMA as permissions can change since migration started" in remove_migration_pte() can result in confusion, because if the source PTE/PMD indicates write permissions, then there should be no need to check VMA write permissions when restoring migration entries or PTE-mapping a PMD. Commit d3cb8bf6 ("mm: migrate: Close race between migration completion and mprotect") introduced the maybe_mkwrite() handling in remove_migration_pte() in 2014, stating that a race between mprotect() and migration finishing would be possible, and that we could end up with a writable PTE that should be readable. However, mprotect() code first updates vma->vm_flags / vma->vm_page_prot and then walks the page tables to (a) set all present writable PTEs to read-only and (b) convert all writable migration entries to readable migration entries. While walking the page tables and modifying the entries, migration code has to grab the PT locks to synchronize against concurrent page table modifications. Assuming migration would find a writable migration entry (while holding the PT lock) and replace it with a writable present PTE, surely mprotect() code didn't stumble over the writable migration entry yet (converting it into a readable migration entry) and would instead wait for the PT lock to convert the now present writable PTE into a read-only PTE. As mprotect() didn't finish yet, the behavior is just like migration didn't happen: a writable PTE will be converted to a read-only PTE. So it's fine to rely on the writability information in the source PTE/PMD and not recheck against the VMA as long as we're holding the PT lock to synchronize with anyone who concurrently wants to downgrade write permissions (like mprotect()) by first adjusting vma->vm_flags / vma->vm_page_prot to then walk over the page tables to adjust the page table entries. Running test cases that should reveal such races -- mprotect(PROT_READ) racing with page migration or THP splitting -- for multiple hours did not reveal an issue with this cleanup. Link: https://lkml.kernel.org/r/20230418142113.439494-1-david@redhat.comSigned-off-by:
David Hildenbrand <david@redhat.com> Acked-by:
Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Reviewed-by:
Alistair Popple <apopple@nvidia.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Peter Xu <peterx@redhat.com> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org>
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