Re: [RFC PATCH 0/6] variable-order, large folios for anonymous memory

[Ryan Roberts <ryan.roberts@arm.com>]

Hi Matthew,

On 17/03/2023 10:57, Ryan Roberts wrote:
> Hi All,
> 
> [...]
> 
> Bug(s)
> ======
> 
> When I run this code without the last (workaround) patch, with DEBUG_VM et al,
> PROVE_LOCKING and KASAN enabled, I see occasional oopses. Mostly these are
> relating to invalid kernel addresses (which usually look like either NULL +
> small offset or mostly zeros with a few mid-order bits set + a small offset) or
> lockdep complaining about a bad unlock balance. Call stacks are often in
> madvise_free_pte_range(), but I've seen them in filesystem code too. (I can
> email example oopses out separately if anyone wants to review them). My hunch is
> that struct pages adjacent to the folio are being corrupted, but don't have hard
> evidence.
> 
> When adding the workaround patch, which prevents madvise_free_pte_range() from
> attempting to split a large folio, I never see any issues. Although I'm not
> putting the system under memory pressure so guess I might see the same types of
> problem crop up under swap, etc.
> 
> I've reviewed most of the code within split_folio() and can't find any smoking
> gun, but I wonder if there are implicit assumptions about the large folio being
> PMD sized that I'm obviously breaking now?
> 
> The code in madvise_free_pte_range():
> 
> 	if (folio_test_large(folio)) {
> 		if (folio_mapcount(folio) != 1)
> 			goto out;
> 		folio_get(folio);
> 		if (!folio_trylock(folio)) {
> 			folio_put(folio);
> 			goto out;
> 		}
> 		pte_unmap_unlock(orig_pte, ptl);
> 		if (split_folio(folio)) {
> 			folio_unlock(folio);
> 			folio_put(folio);
> 			orig_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> 			goto out;
> 		}
> 		...
> 	}

I've noticed that its folio_split() with a folio order of 1 that causes my
problems. And I also see that the page cache code always explicitly never
allocates order-1 folios:

void page_cache_ra_order(struct readahead_control *ractl,
		struct file_ra_state *ra, unsigned int new_order)
{
	...

	while (index <= limit) {
		unsigned int order = new_order;

		/* Align with smaller pages if needed */
		if (index & ((1UL << order) - 1)) {
			order = __ffs(index);
			if (order == 1)
				order = 0;
		}
		/* Don't allocate pages past EOF */
		while (index + (1UL << order) - 1 > limit) {
			if (--order == 1)
				order = 0;
		}
		err = ra_alloc_folio(ractl, index, mark, order, gfp);
		if (err)
			break;
		index += 1UL << order;
	}

	...
}

Matthew, what is the reason for this? I suspect its guarding against the same
problem I'm seeing.

If I explicitly prevent order-1 allocations for anon pages, I'm unable to cause
any oops/panic/etc. I'd just like to understand the root cause.

Thanks,
Ryan



> 
> Will normally skip my large folios because they have a mapcount > 1, due to
> incrementing mapcount for each pte, unlike PMD mapped pages. But on occasion it
> will see a mapcount of 1 and proceed. So I guess this is racing against reclaim
> or CoW in this case?
> 
> I also see its doing a dance to take the folio lock and drop the ptl. Perhaps my
> large anon folio is not using the folio lock in the same way as a THP would and
> we are therefore not getting the expected serialization?
> 
> I'd really appreciate any suggestions for how to pregress here!
>