On Wed, 2022-04-13 at 09:26 +0800, ying.huang@xxxxxxxxx wrote: > On Tue, 2022-04-12 at 16:59 +0200, David Hildenbrand wrote: > > On 12.04.22 15:42, Miaohe Lin wrote: > > > On 2022/4/12 16:59, Oscar Salvador wrote: > > > > On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote: > > > > > We don't need to check can_split_folio() because folio_maybe_dma_pinned() > > > > > is checked before. It will avoid the long term pinned pages to be swapped > > > > > out. And we can live with short term pinned pages. Without can_split_folio > > > > > checking we can simplify the code. Also activate_locked can be changed to > > > > > keep_locked as it's just short term pinning. > > > > > > > > What do you mean by "we can live with short term pinned pages"? > > > > Does it mean that it was not pinned when we check > > > > folio_maybe_dma_pinned() but now it is? > > > > > > > > To me it looks like the pinning is fluctuating and we rely on > > > > split_folio_to_list() to see whether we succeed or not, and if not > > > > we give it another spin in the next round? > > > > > > Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be > > > pinned for a noticeable time. So it's expected to split the folio successfully in the next > > > round as the pinning is really fluctuating. Or am I miss something? > > > > > > > Just so we're on the same page. folio_maybe_dma_pinned() only capture > > FOLL_PIN, but not FOLL_GET. You can have long-term FOLL_GET right now > > via vmsplice(). > > Per my original understanding, folio_maybe_dma_pinned() can be used to > detect long-term pinned pages. And it seems reasonable to skip the > long-term pinned pages and try short-term pinned pages during page > reclaiming. But as you pointed out, vmsplice() doesn't use FOLL_PIN. > So if vmsplice() is expected to pin pages for long time, and we have no > way to detect it, then we should keep can_split_folio() in the original > code. > > Copying more people who have worked on long-term pinning for comments. Checked the discussion in the following thread, https://lore.kernel.org/lkml/CA+CK2bBffHBxjmb9jmSKacm0fJMinyt3Nhk8Nx6iudcQSj80_w@xxxxxxxxxxxxxx/ It seems that from the practical point of view, folio_maybe_dma_pinned() can identify most long-term pinned pages that may block memory hot- remove or CMA allocation. Although as David pointed out, some pages may still be GUPed for long time (e.g. via vmsplice) even if !folio_maybe_dma_pinned(). But from another point of view, can_split_huge_page() is cheap and THP swapout is expensive (swap space, disk IO, and hard to be recovered), so it may be better to keep can_split_huge_page() in shink_page_list(). Best Regards, Huang, Ying > > > can_split_folio() is more precise then folio_maybe_dma_pinned(), but > > both are racy as long as the page is still mapped. > > > > >
