On Thu, Jun 25, 2020 at 03:40:44PM +0200, Jan Kara wrote: > On Thu 25-06-20 12:42:09, Matthew Wilcox wrote: > > Why are DMA pinned pages still on the LRU list at all? I never got an > > answer to this that made sense to me. By definition, a page which is > > pinned for DMA is being accessed, and needs to at the very least change > > position on the LRU list, so just take it off the list when DMA-pinned > > and put it back on the list when DMA-unpinned. > > Well, we do mark_page_accessed() when pinning in GUP. This is not perfect > but it's as good as it gets with CPU having no control when the page is > actually accessed. Also taking the page off and then back to LRU list would > increase the contention on the LRU list locks and generally cost > performance so for short term pins it is not desirable... Otherwise I agree > that conceptually it would make some sence although I'm not sure some > places wouldn't get confused by e.g. page cache pages not being on LRU > list. We could/should do what we do for mlocked pages: Documentation/vm/unevictable-lru.rst I think 'case five' is wrong and needs to be removed. Pinning is inappropriate for "I'm going to modify the page myself".
