Messages in this thread | | | Subject | Re: [PATCH v2 1/8] mm: dump_page: print head page's refcount, for compound pages | From | John Hubbard <> | Date | Wed, 29 Jan 2020 22:30:56 -0800 |
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On 1/29/20 10:23 PM, John Hubbard wrote: > On 1/29/20 2:59 PM, Matthew Wilcox wrote: > ... >> I have a hunk in my current tree which looks like this: >> >> @@ -77,6 +77,11 @@ void __dump_page(struct page *page, const char *reason) >> pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx >> \n", >> page, page_ref_count(page), mapcount, >> page->mapping, page_to_pgoff(page)); >> + if (PageTail(page)) { >> + struct page *head = compound_head(page); >> + pr_warn("head:%px mapping:%px index:%#lx\n", >> + head, head->mapping, page_to_pgoff(head)); >> + } >> if (PageKsm(page)) >> pr_warn("ksm flags: %#lx(%pGp)\n", page->flags, &page->flags); >> else if (PageAnon(page)) >> >> I wonder if we can combine these two patches in some more useful way? >> >> I also think we probably want a sanity check that 'head' and 'page' >> are within a sane range of each other (ie head < page and head + >> MAX_ORDER_NR_PAGES > page) to protect against a struct page that contains >> complete garbage. >> > > OK, here's a go at combining those. I like the observation, implicit in your > diffs, that PageTail rather than PageCompound is the key differentiator in > deciding what to print. How's this look: > > diff --git a/mm/debug.c b/mm/debug.c > index a90da5337c14..944652843e7b 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -75,12 +75,31 @@ void __dump_page(struct page *page, const char *reason) > */ > mapcount = PageSlab(page) ? 0 : page_mapcount(page); > > - if (PageCompound(page)) > - pr_warn("page:%px refcount:%d mapcount:%d mapping:%px " > - "index:%#lx compound_mapcount: %d\n", > - page, page_ref_count(page), mapcount, > - page->mapping, page_to_pgoff(page), > - compound_mapcount(page)); > + if (PageTail(page)) { > + struct page *head = compound_head(page); > + > + if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) { > + /* > + * Page is hopelessly corrupted, so limit any reporting > + * to information about the page itself. Do not attempt > + * to look at the head page. > + */ > + pr_warn("page:%px refcount:%d mapcount:%d mapping:%px " > + "index:%#lx (corrupted tail page case)\n", > + page, page_ref_count(page), mapcount, > + page->mapping, page_to_pgoff(page)); > + } else { > + pr_warn("page:%px compound refcount:%d mapcount:%d " > + "mapping:%px index:%#lx compound_mapcount:%d\n", > + page, page_ref_count(head), > + mapcount, head->mapping, page_to_pgoff(head), > + compound_mapcount(page)); > + > + if (page_ref_count(page) != 0) > + pr_warn("page:%px PROBLEM: non-zero refcount (==%d) on " > + "this tail page\n", page, page_ref_count(page));
...ahem, I sorta botched the above statement, because that should be outside (just below) the "else" statement--it can be done whether or not the page fails the safety/bounds check. :)
thanks, -- John Hubbard NVIDIA
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