On 1/10/22 05:37, Matthew Wilcox wrote: > On Mon, Jan 10, 2022 at 12:29:58AM -0800, Christoph Hellwig wrote: >> On Mon, Jan 10, 2022 at 04:23:44AM +0000, Matthew Wilcox (Oracle) wrote: >>> Several functions in gup.c assume that a compound page has virtually >>> contiguous page structs. This isn't true for SPARSEMEM configs unless >>> SPARSEMEM_VMEMMAP is also set. Fix them by using nth_page() instead of >>> plain pointer arithmetic. >> >> So is this an actualy bug that need a Fixes tag, or do all architectures >> that support THP and sparsemem use SPARSEMEM_VMEMMAP? > > As far as I can tell (and I am by no means an expert in this area), > this problem only affects pages of order MAX_ORDER or higher. That is, > somebody using regular 2MB hugepages on x86 won't see a problem, whether > they're using VMEMMAP or not. It only starts to become a problem for > 1GB hugepages. > > Since THPs are (currently) only allocated from the page allocator, it's > never a problem for THPs, only hugetlbfs. Correcting the places which > can't see a 1GB page is just defense against copy-and-paste programming. > > So I'll defer to Mike -- does this ever affect real systems and thus > warrant a backport? I know this doesn't affect UEK because we enable > SPARSEMEM_VMEMMAP. I guess it all depends on your definition of 'real' systems. I am unaware of any distros that disable SPARSEMEM_VMEMMAP, but I do not know or have access to them all. In arch specific Kconfig files, SPARSEMEM_VMEMMAP is enabled by default (if sparsemem is enabled). However, it is 'possible' to configure a kernel with SPARSEMEM and without SPARSEMEM_VMEMMAP. This issue came up almost a year ago in this thread: https://lore.kernel.org/linux-mm/20210217184926.33567-1-mike.kravetz@xxxxxxxxxx/ In practice, I do not recall ever seeing this outside of debug environments specifically trying to hit the issue. -- Mike Kravetz
