On Tue, Jan 19, 2016 at 09:46:21PM +0900, Tetsuo Handa wrote:
> Kirill A. Shutemov wrote:
> > Oh. Looks like a bug from 2013...
> >
> > Thanks for report.
> >
> > From 6f80a79dc5f65f29899e396942d40f727cd36480 Mon Sep 17 00:00:00 2001
> > From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Date: Tue, 19 Jan 2016 14:59:19 +0300
> > Subject: [PATCH] mm: fix mlock accouting
> >
> > Tetsuo Handa reported underflow of NR_MLOCK on munlock.
> >
> > Testcase:
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <sys/mman.h>
> >
> > #define BASE ((void *)0x400000000000)
> > #define SIZE (1UL << 21)
> >
> > int main(int argc, char *argv[])
> > {
> > void *addr;
> >
> > system("grep Mlocked /proc/meminfo");
> > addr = mmap(BASE, SIZE, PROT_READ | PROT_WRITE,
> > MAP_ANONYMOUS | MAP_PRIVATE | MAP_LOCKED | MAP_FIXED,
> > -1, 0);
> > if (addr == MAP_FAILED)
> > printf("mmap() failed\n"), exit(1);
> > munmap(addr, SIZE);
> > system("grep Mlocked /proc/meminfo");
> > return 0;
> > }
> >
> > It happens on munlock_vma_page() due to unfortunate choice of nr_pages
> > data type:
> >
> > __mod_zone_page_state(zone, NR_MLOCK, -nr_pages);
> >
> > For unsigned int nr_pages, implicitly casted to long in
> > __mod_zone_page_state(), it becomes something around UINT_MAX.
> >
> > munlock_vma_page() usually called for THP as small pages go though
> > pagevec.
> >
> > Let's make nr_pages singed int.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Fixes: ff6a6da60b89 ("mm: accelerate munlock() treatment of THP pages")
> > Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Cc: Michel Lespinasse <walken@xxxxxxxxxx>
> > ---
> > mm/mlock.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/mlock.c b/mm/mlock.c
> > index e1e2b1207bf2..96f001041928 100644
> > --- a/mm/mlock.c
> > +++ b/mm/mlock.c
> > @@ -175,7 +175,7 @@ static void __munlock_isolation_failed(struct page *page)
> > */
> > unsigned int munlock_vma_page(struct page *page)
> > {
> > - unsigned int nr_pages;
> > + int nr_pages;
> > struct zone *zone = page_zone(page);
> >
> > /* For try_to_munlock() and to serialize with page migration */
> > --
> > Kirill A. Shutemov
> >
>
> Don't we want to use "long" than "int" for all variables that count number
> of pages, for recently commit 6cdb18ad98a49f7e9b95d538a0614cde827404b8
> "mm/vmstat: fix overflow in mod_zone_page_state()" changed to use "long" ?
Potentially, yes. But here we count number of small pages in the compound
page. We're far from being able to allocate 8 terabyte pages ;)
Anyway, it's out-of-scope for this bug fix.
My "Fixes:" is probably misleading, since we don't have bug visible until
6cdb18ad98a4.
--
Kirill A. Shutemov
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