Messages in this thread Patch in this message | | | Subject | Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone | From | Yang Shi <> | Date | Fri, 25 Oct 2019 08:58:22 -0700 |
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On 10/25/19 8:36 AM, Kirill A. Shutemov wrote: > On Fri, Oct 25, 2019 at 01:27:46AM +0800, Yang Shi wrote: >> File THP sets PageDoubleMap flag when the first it gets PTE mapped, but >> the flag is never cleared until the THP is freed. This result in >> unbalanced state although it is not a big deal. >> >> Clear the flag when the last compound_mapcount is gone. It should be >> cleared when all the PTE maps are gone (become PMD mapped only) as well, >> but this needs check all subpage's _mapcount every time any subpage's >> rmap is removed, the overhead may be not worth. The anonymous THP also >> just clears PageDoubleMap flag when the last PMD map is gone. > NAK, sorry. > > The key difference with anon THP that file THP can be mapped again with > PMD after all PMD (or all) mappings are gone. > > Your patch breaks the case when you map the page with PMD again while the > page is still mapped with PTEs. Who would set PageDoubleMap() in this > case?
Aha, yes, you are right. I missed that point. However, I'm wondering we might move this up a little bit like this:
diff --git a/mm/rmap.c b/mm/rmap.c index d17cbf3..ac046fd 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1230,15 +1230,17 @@ static void page_remove_file_rmap(struct page *page, bool compound) if (atomic_add_negative(-1, &page[i]._mapcount)) nr++; } + + /* No PTE map anymore */ + if (nr == HPAGE_PMD_NR) + ClearPageDoubleMap(compound_head(page)); + if (!atomic_add_negative(-1, compound_mapcount_ptr(page))) goto out; if (PageSwapBacked(page)) __dec_node_page_state(page, NR_SHMEM_PMDMAPPED); else __dec_node_page_state(page, NR_FILE_PMDMAPPED); - - /* The last PMD map is gone */ - ClearPageDoubleMap(compound_head(page)); } else { if (!atomic_add_negative(-1, &page->_mapcount)) goto out;
This should guarantee no PTE map anymore, it should be safe to clear the flag.
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