| Message ID | 20200201034029.4063170-2-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/gup: track FOLL_PIN pages | expand |
On Fri, Jan 31, 2020 at 07:40:18PM -0800, John Hubbard wrote: > A compound page collects the refcount in the head page, while leaving > the refcount of each tail page at zero. Therefore, when debugging a > problem that involves compound pages, it's best to have diagnostics that > reflect that situation. However, dump_page() is oblivious to these > points. > > Change dump_page() as follows: > > 1) For tail pages, print relevant head page information: refcount, in > particular. But only do this if the page is not corrupted so badly > that the pointer to the head page is all wrong. > > 2) Do a separate check to catch any (rare) cases of the tail page's > refcount being non-zero, and issue a separate, clear pr_warn() if > that ever happens. > > Suggested-by: Matthew Wilcox <willy@infradead.org> > Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Few nit-picks below. > --- > mm/debug.c | 34 ++++++++++++++++++++++++++++------ > 1 file changed, 28 insertions(+), 6 deletions(-) > > diff --git a/mm/debug.c b/mm/debug.c > index ecccd9f17801..beb1c59d784b 100644 > --- a/mm/debug.c > +++ b/mm/debug.c > @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = { > {0, NULL} > }; > > +static void __dump_tail_page(struct page *page, int mapcount) > +{ > + struct page *head = compound_head(page); > + > + if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) { I'm not sure if we want to use compound_nr() here instead of MAX_ORDER_NR_PAGES. Do you have any reasonaing about it? > + /* > + * 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)); Wrap into {}, please. > +} > + > void __dump_page(struct page *page, const char *reason) > { > struct address_space *mapping; > @@ -75,12 +101,8 @@ 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)) > + __dump_tail_page(page, mapcount); > else > pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n", > page, page_ref_count(page), mapcount, > -- > 2.25.0 >
On 2/3/20 5:16 AM, Kirill A. Shutemov wrote: > On Fri, Jan 31, 2020 at 07:40:18PM -0800, John Hubbard wrote: >> A compound page collects the refcount in the head page, while leaving >> the refcount of each tail page at zero. Therefore, when debugging a >> problem that involves compound pages, it's best to have diagnostics that >> reflect that situation. However, dump_page() is oblivious to these >> points. >> >> Change dump_page() as follows: >> >> 1) For tail pages, print relevant head page information: refcount, in >> particular. But only do this if the page is not corrupted so badly >> that the pointer to the head page is all wrong. >> >> 2) Do a separate check to catch any (rare) cases of the tail page's >> refcount being non-zero, and issue a separate, clear pr_warn() if >> that ever happens. >> >> Suggested-by: Matthew Wilcox <willy@infradead.org> >> Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> >> Signed-off-by: John Hubbard <jhubbard@nvidia.com> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Thanks for looking through all of these! > > Few nit-picks below. > >> --- >> mm/debug.c | 34 ++++++++++++++++++++++++++++------ >> 1 file changed, 28 insertions(+), 6 deletions(-) >> >> diff --git a/mm/debug.c b/mm/debug.c >> index ecccd9f17801..beb1c59d784b 100644 >> --- a/mm/debug.c >> +++ b/mm/debug.c >> @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = { >> {0, NULL} >> }; >> >> +static void __dump_tail_page(struct page *page, int mapcount) >> +{ >> + struct page *head = compound_head(page); >> + >> + if ((page < head) || (page >= head + MAX_ORDER_NR_PAGES)) { > > I'm not sure if we want to use compound_nr() here instead of > MAX_ORDER_NR_PAGES. Do you have any reasonaing about it? Yes: compound_nr(page) reads from the struct page, whereas MAX_ORDER_NR_PAGES is an independent, immutable limit. When checking a struct page for corruption, it's ideal to avoid relying on data within the struct page, as compound_nr() would have to do. > >> + /* >> + * 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)); > > Wrap into {}, please. Fixed, thanks. thanks,
diff --git a/mm/debug.c b/mm/debug.c index ecccd9f17801..beb1c59d784b 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -42,6 +42,32 @@ const struct trace_print_flags vmaflag_names[] = { {0, NULL} }; +static void __dump_tail_page(struct page *page, int mapcount) +{ + 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)); +} + void __dump_page(struct page *page, const char *reason) { struct address_space *mapping; @@ -75,12 +101,8 @@ 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)) + __dump_tail_page(page, mapcount); else pr_warn("page:%px refcount:%d mapcount:%d mapping:%px index:%#lx\n", page, page_ref_count(page), mapcount,
A compound page collects the refcount in the head page, while leaving the refcount of each tail page at zero. Therefore, when debugging a problem that involves compound pages, it's best to have diagnostics that reflect that situation. However, dump_page() is oblivious to these points. Change dump_page() as follows: 1) For tail pages, print relevant head page information: refcount, in particular. But only do this if the page is not corrupted so badly that the pointer to the head page is all wrong. 2) Do a separate check to catch any (rare) cases of the tail page's refcount being non-zero, and issue a separate, clear pr_warn() if that ever happens. Suggested-by: Matthew Wilcox <willy@infradead.org> Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- mm/debug.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-)