On Tue, Sep 22, 2020 at 04:12:15AM +0100, Matthew Wilcox wrote: > This is a fun little race. > > Dramatis personae: Pages P0, P1 are consecutive and aligned. > Threads A, B, C. > > Page P0 is allocated to the page cache. > Page P1 is free. > > Thread A calls find_get_entry() > P0 is returned from xas_load() > > Thread B removes the page from the page cache (eg truncate, invalidatepage). > P0 is buddy-merged with P1. > > Thread C calls alloc_pages, order 1, does not specify GFP_COMP. P0 now > has refcount 1. > > Thread A calls page_cache_get_speculative(). P0 has refcount 2. > > Thread C calls __free_page(P0, 1) > put_page_testzero is _false_. Do not call free_the_page(). > > Thread A calls put_page(P0) > We free P0 and nobody knows to free P1. > > > Weird solution: In __free_page(), if put_page_testzero() fails and page > is not PageHead, I'm not an expert, so I'm probably off here, but if Thread C calls __free_page(P0, 1) wouldn't that be on PageHead? So how would you know to convert it to a compound page? Ira > convert it to a compound page. Then the put_page() > by Thread A will free P1. > > Better ideas? >
