On 29 Oct 2020, at 17:54, Matthew Wilcox wrote: > On Thu, Oct 29, 2020 at 04:49:39PM -0400, Zi Yan wrote: >> On 29 Oct 2020, at 15:33, Matthew Wilcox (Oracle) wrote: >> >>> We currently store order-N THPs as 2^N consecutive entries. While this >>> consumes rather more memory than necessary, it also turns out to be buggy. >>> A writeback operation which starts in the middle of a dirty THP will not >>> notice as the dirty bit is only set on the head index. With multi-index >>> entries, the dirty bit will be found no matter where in the THP the >>> iteration starts. >> >> A multi-index entry can point to a THP with any size and the code relies >> on thp_last_tail() to check whether it has finished processing the page >> pointed by the entry. Is it how this change works? > > Maybe I need to do a better explanation here. Let me try again ... > > Consider an order-2 page (at address p) at index 4. Before this change, > the node in the XArray contains: > > 4: p > 5: p > 6: p > 7: p > > After this change, it contains: > > 4: p > 5: sibling(4) > 6: sibling(4) > 7: sibling(4) > > When we mark page p as dirty, we set a bit on entry 4, since that's the > head page. Now we try to fsync pages 5-19, we start the lookup at index 5. > Before this patch, the pagecache knows that p is a head page, but the > XArray doesn't. So when it looks at entry 5, it sees a normal pointer > and no mark on it -- the XArray doesn't get to interpret the contents > of the pointers stored in it. After this patch, we tell the XArray that > indices 4-7 are a single entry, so the marked iteration actually loads > the entry at 5, sees it's a sibling of 4, sees that 4 is marked dirty > and returns p. Got it. Thanks for the explanation. Could you include this in the commit message? — Best Regards, Yan Zi
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