Re: Known and unfixed active data loss bug in MM + XFS with large folios since Dec 2021 (any kernel from 6.1 upwards)

Jens Axboe
2024-09-19T04:46:10Z

On 9/18/24 10:36 PM, Matthew Wilcox wrote:
> On Wed, Sep 18, 2024 at 09:38:41PM -0600, Jens Axboe wrote:
>> On 9/18/24 9:12 PM, Linus Torvalds wrote:
>>> On Thu, 19 Sept 2024 at 05:03, Linus Torvalds
>>> <torvalds@linux-foundation.org> wrote:
>>>>
>>>> I think we should just do the simple one-liner of adding a
>>>> "xas_reset()" to after doing xas_split_alloc() (or do it inside the
>>>> xas_split_alloc()).
>>>
>>> .. and obviously that should be actually *verified* to fix the issue
>>> not just with the test-case that Chris and Jens have been using, but
>>> on Christian's real PostgreSQL load.
>>>
>>> Christian?
>>>
>>> Note that the xas_reset() needs to be done after the check for errors
>>> - or like Willy suggested, xas_split_alloc() needs to be re-organized.
>>>
>>> So the simplest fix is probably to just add a
>>>
>>> if (xas_error(&xas))
>>> goto error;
>>> }
>>> + xas_reset(&xas);
>>> xas_lock_irq(&xas);
>>> xas_for_each_conflict(&xas, entry) {
>>> old = entry;
>>>
>>> in __filemap_add_folio() in mm/filemap.c
>>>
>>> (The above is obviously a whitespace-damaged pseudo-patch for the
>>> pre-6758c1128ceb state. I don't actually carry a stable tree around on
>>> my laptop, but I hope it's clear enough what I'm rambling about)
>>
>> I kicked off a quick run with this on 6.9 with my debug patch as well,
>> and it still fails for me... I'll double check everything is sane. For
>> reference, below is the 6.9 filemap patch.
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 30de18c4fd28..88093e2b7256 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -883,6 +883,7 @@ noinline int __filemap_add_folio(struct address_space *mapping,
>> if (order > folio_order(folio))
>> xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index),
>> order, gfp);
>> + xas_reset(&xas);
>> xas_lock_irq(&xas);
>> xas_for_each_conflict(&xas, entry) {
>> old = entry;
>
> My brain is still mushy, but I think there is still a problem (both with
> the simple fix for 6.9 and indeed with 6.10).
>
> For splitting a folio, we have the folio locked, so we know it's not
> going anywhere. The tree may get rearranged around it while we don't
> have the xa_lock, but we're somewhat protected.
>
> In this case we're splitting something that was, at one point, a shadow
> entry. There's no struct there to lock. So I think we can have a
> situation where we replicate 'old' (in 6.10) or xa_load() (in 6.9)
> into the nodes we allocate in xas_split_alloc(). In 6.10, that's at
> least guaranteed to be a shadow entry, but in 6.9, it might already be a
> folio by this point because we've raced with something else also doing a
> split.
>
> Probably xas_split_alloc() needs to just do the alloc, like the name
> says, and drop the 'entry' argument. ICBW, but I think it explains
> what you're seeing? Maybe it doesn't?

Since I can hit it pretty reliably and quickly, I'm happy to test
whatever you want on top of 6.9. From the other email, I backported:

a4864671ca0b ("lib/xarray: introduce a new helper xas_get_order")
6758c1128ceb ("mm/filemap: optimize filemap folio adding")

to 6.9 and kicked off a test with that 5 min ago, and it's still going.
I'd say with 90% confidence that it should've hit already, but let's
leave it churning for an hour and see what pops out the other end.

--
Jens Axboe