On Thu, Jul 27, 2023 at 8:18 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote: > > This allows batching the rmap removal with folio_remove_rmap_range(), > which means we avoid spuriously adding a partially unmapped folio to the > deferred split queue in the common case, which reduces split queue lock > contention. > > Previously each page was removed from the rmap individually with > page_remove_rmap(). If the first page belonged to a large folio, this > would cause page_remove_rmap() to conclude that the folio was now > partially mapped and add the folio to the deferred split queue. But > subsequent calls would cause the folio to become fully unmapped, meaning > there is no value to adding it to the split queue. > > A complicating factor is that for platforms where MMU_GATHER_NO_GATHER > is enabled (e.g. s390), __tlb_remove_page() drops a reference to the > page. This means that the folio reference count could drop to zero while > still in use (i.e. before folio_remove_rmap_range() is called). This > does not happen on other platforms because the actual page freeing is > deferred. > > Solve this by appropriately getting/putting the folio to guarrantee it > does not get freed early. Given the need to get/put the folio in the > batch path, we stick to the non-batched path if the folio is not large. > While the batched path is functionally correct for a folio with 1 page, > it is unlikely to be as efficient as the existing non-batched path in > this case. > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> This ad hoc patch looks unacceptable to me: we can't afford to keep adding special cases. I vote for completely converting zap_pte_range() to use folio_remove_rmap_range(), and that includes tlb_flush_rmap_batch() and other types of large folios, not just anon. Otherwise I'll leave it to Matthew and David.
