On Mon, Mar 1, 2021 at 11:59 PM Ilya Lipnitskiy
<ilya.lipnitskiy@xxxxxxxxx> wrote:
>
> Good to know. Some more digging and I can say that we hit this error
> when trying to unmap PFN 0 (is_zero_pfn(pfn) returns TRUE,
> vm_normal_page returns NULL, zap_pte_range does not decrement
> MM_ANONPAGES RSS counter). Is my understanding correct that PFN 0 is
> usable, but special? Or am I totally off the mark here?
PFN 0 should be usable - depending on architecture, of course - and
shouldn't even be special in any way.
is_zero_pfn(pfn) is *not* meant to test for pfn being 0 - it's meant
to test for the pfn pointing to the special zero-filled page. The two
_could_ be the same thing, of course, but generally are not (h8300
seems to say "we use pfn 0 as the zero page" if I read things right).
In fact, there can be many zero-filled pages - architectures with
virtually mapped caches that want cache coloring have multiple
contiguous zero-filled pages and then map in the right one based on
virtual address. I'm not sure why it would matter (the zero-page is
always mapped read-only, so any physical aliases should be a
non-issue), but whatever..
> Here is the (optimized) stack trace when the counter does not get decremented:
> [<8015b078>] vm_normal_page+0x114/0x1a8
Yes, if "is_zero_pfn()" returns true, then it won't be considered a
normal page, and is not refcounted.
But that should only trigger for pfn == zero_pfn, and zero_pfn should
be initialized to
zero_pfn = page_to_pfn(ZERO_PAGE(0));
so it _sounds_ like you possibly have something odd going on with ZERO_PAGE.
Yes, one architecture does actually make pfn 0 _be_ the zero page, but
you said MIPS, and that does do the page coloring games, and has
#define ZERO_PAGE(vaddr) \
(virt_to_page((void *)(empty_zero_page + (((unsigned
long)(vaddr)) & zero_page_mask))))
where zero_page_mask is the page colorign mask, and empty_zero_page is
allocated in setup_zero_pages() fairly early in mem_init() (again, it
allocates multiple pages depending on the page ordering - see that
horrible virtual cache thing with cpu_has_vce).
So PFN 0 shouldn't be an issue at all.
Of course, since you said this was an embedded MIPS platform, maybe
it's one of the broken ones with virtual caches and cpu_has_vce is
set. I'm not sure how much testing that has gotten lately. MOST of the
later MIPS architectures walked away from the pure virtual cache
setups.
Linus
