On 27/07/2023 18:22, Yu Zhao wrote: > 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. The intent of the change is to avoid the deferred split queue lock contention, and this is only a problem for anon folios; page cache folios are never split in this way. My intention was to do the smallest change to solve the problem. I don't see the value in reworking a much bigger piece of the code, making it more complex, when its not going to give any clear perf benefits. Otherwise I'll leave > it to Matthew and David. If there is concensus that this is _required_ in order to merge this series, then I guess I'll bite the bullet and do it. But my preference is to leave it for if/when a reason is found that it is actually bringing benefit.
