On 30 Mar 2022, at 19:48, Zi Yan wrote: > 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? OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no problem with PageBuddy(0x21ee00). -- Best Regards, Yan, Zi
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