"David Hildenbrand (Arm)" <[email protected]> writes: > On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote: >> From: Ackerley Tng <[email protected]> >> >> 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. > > As discussed, what would happen if we mlocked gmem folios? In that case, they > could also have been added to the mlock cache (which we treat like an LRU > cache). > > But that's just a comment regarding the text here. > > [...] >
To make sure I'll get the wording right, let me know what you think of this (will replace the entire commit message for v11): 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. A folio may also have extra refcounts if it is on the mlock fbatch. These two known "usages" of the folio are handled calling lru_cache_drain_for_folio, which drains both the lru_add and mlock fbatches. 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. > -- > Cheers, > > David
