On Fri, Jul 10, 2020 at 2:35 AM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> wrote: > > 在 2020/7/10 下午1:28, Mika Penttilä 写道: > > > > > > On 10.7.2020 7.51, Alex Shi wrote: > >> > >> 在 2020/7/10 上午12:07, Kirill A. Shutemov 写道: > >>> On Thu, Jul 09, 2020 at 04:50:02PM +0100, Matthew Wilcox wrote: > >>>> On Thu, Jul 09, 2020 at 11:11:11PM +0800, Alex Shi wrote: > >>>>> Hi Kirill & Matthew, > >>>>> > >>>>> In the func call chain, from split_huge_page() to lru_add_page_tail(), > >>>>> Seems tail pages are added to lru list at line 963, but in this scenario > >>>>> the head page has no lru bit and isn't set the bit later. Why we do this? > >>>>> or do I miss sth? > >>>> I don't understand how we get to split_huge_page() with a page that's > >>>> not on an LRU list. Both anonymous and page cache pages should be on > >>>> an LRU list. What am I missing?> > >> > >> Thanks a lot for quick reply! > >> What I am confusing is the call chain: __iommu_dma_alloc_pages() > >> to split_huge_page(), in the func, splited page, > >> page = alloc_pages_node(nid, alloc_flags, order); > >> And if the pages were added into lru, they maybe reclaimed and lost, > >> that would be a panic bug. But in fact, this never happened for long time. > >> Also I put a BUG() at the line, it's nevre triggered in ltp, and run_vmtests > > > > > > In __iommu_dma_alloc_pages, after split_huge_page(), who is taking a > > reference on tail pages? Seems tail pages are freed and the function > > errornously returns them in pages[] array for use? > > > > CC Joerg and iommu list, > > That's a good question. seems the split_huge_page was never triggered here, > since the func would check the PageLock first. and have page->mapping and PageAnon > check, any of them couldn't be matched for the alloced page. > > Hi Joerg, > would you like look into this? do we still need the split_huge_page() here? I think this is the same problem which has been discussed a couple of weeks ago. Please refer to: https://lore.kernel.org/linux-mm/20200619001938.GA135965@xxxxxxxxxxxxxxxxxxxxxxxxxxx/ I think the conclusion is split_huge_page() can't be used in this path at all. But we didn't reach a fix yet. > > Thanks > Alex > > int split_huge_page_to_list(struct page *page, struct list_head *list) > { > struct page *head = compound_head(page); > struct deferred_split *ds_queue = get_deferred_split_queue(head); > struct anon_vma *anon_vma = NULL; > struct address_space *mapping = NULL; > int count, mapcount, extra_pins, ret; > pgoff_t end; > > VM_BUG_ON_PAGE(is_huge_zero_page(head), head); > VM_BUG_ON_PAGE(!PageLocked(head), head); <== > > >
