On 30 Mar 2022, at 19:03, Linus Torvalds wrote: > On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <zi.yan@xxxxxxxx> wrote: >> >> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others") > > Oh, btw - should this perhaps be backported further back than that > alleged "fixes" commit? > > It does look like maybe the problem potentially existed before too, > and was just much harder to trigger. > > That said, google doesn't find any other reports that look like > Steven's oops, so maybe it really never happened and backporting isn't > called for. > > Or possibly my google-fu is just bad. > There might not be any issue with the original code because this bug could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION are both set, which never happens, since CONFIG_MEMORY_ISOLATION depends on CONFIG_SPARSEMEM. By checking Steven's boot log, it should be PFN 0x21ee00 that triggers the bug, since the physical memory range ends at PFN 0x21edff. PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned. The original code assumes all physical memory ranges are at least MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM allocates pageblock_flags array (the NULL-deferenced bitmap points to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM does not do this. It allocates pageblock_flags array at the exact size of the physical memory. So checking 0x21ee00 will not cause NULL dereferencing when CONFIG_MEMORY_ISOLATION is set and the original if statement can be true. Now I am wondering if the page_is_buddy() check is correct for CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1 or just the present physical memory range? Is PageBuddy(0x21ee00) accessing some random memory location? -- Best Regards, Yan, Zi
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