Re: [PATCH v10 15/41] KVM: guest_memfd: Handle lru_add fbatch refcounts during conversion safety check
From: Binbin Wu <hidden>
Date: 2026-08-14 03:26:02
Also in:
kvm, linux-coco, linux-doc, linux-kselftest, linux-mm, lkml
On 8/8/2026 5:52 AM, Ackerley Tng via B4 Relay wrote:
From: Ackerley Tng <redacted> A guest_memfd folio is safe for conversion if guest_memfd holds the last references on it. Any other references on the folio may indicate another user, and guest_memfd cannot convert it to private if there may be an existing host user. A folio will have extra refcounts if it is present in a per-CPU lru_add fbatch. guest_memfd does not actually participate in LRU, but freshly-allocated folios are still added to the lru_add fbatch for batch LRU statistics processing. This one known "usage" of the folio is handled by draining the lru_add fbatch. After draining, if the refcount is still elevated, then there's truly some other user of this page, and the page is not safe for conversion. If the page may be dma pinned, DMA is obviously using it and hence not safe for conversions. If the page is still mapped after guest_memfd tried to unmap it earlier in the conversion process, it is also obviously not safe for conversion. Exit early to avoid unnecessary draining in these 2 cases. Provide a drain status to only drain once ever while processing a batch of folios. Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org> Suggested-by: David Hildenbrand <david@kernel.org> Signed-off-by: Ackerley Tng <redacted>
With the updated version of change log, Reviewed-by: Binbin Wu <redacted> One nit below. [...]
+static bool __folio_safe_for_conversion(struct folio *folio,
+ enum lru_cache_drained *drained)
+{
+ const int filemap_get_folios_refcount = 1;
+Nit: After adding the helper, the use of filemap_get_folios_refcount seems less obvious. It introduces unnecessary tight coupling with the caller, though it has only one caller currently. Is it better to pass the value as extra_refs, similar to what lru_cache_drain_for_folio() does?
quoted hunk ↗ jump to hunk
+ if (folio_maybe_dma_pinned(folio) || folio_mapped(folio)) + return false; + + lru_cache_drain_for_folio(folio, filemap_get_folios_refcount, + drained); + + return folio_ref_count(folio) == + folio_nr_pages(folio) + filemap_get_folios_refcount; +} + static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start, size_t nr_pages, pgoff_t *err_index) { + enum lru_cache_drained drained = LRU_CACHE_NOT_DRAINED; struct address_space *mapping = inode->i_mapping; - const int filemap_get_folios_refcount = 1; pgoff_t last = start + nr_pages - 1; struct folio_batch fbatch; bool safe = true;@@ -560,9 +576,8 @@ static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start, for (i = 0; i < folio_batch_count(&fbatch); ++i) { struct folio *folio = fbatch.folios[i]; - if (folio_ref_count(folio) != - folio_nr_pages(folio) + filemap_get_folios_refcount) { - safe = false; + safe = __folio_safe_for_conversion(folio, &drained); + if (!safe) { *err_index = max(start, folio->index); break; }