Re: [PATCH resend] ramdisk: fix zeroed ramdisk pages on memory pressure

[ebiederm@xmission.com (Eric W. Biederman)]

Nick Piggin <nickpiggin@yahoo.com.au> writes:

> On Monday 15 October 2007 19:05, Christian Borntraeger wrote:
>> Am Montag, 15. Oktober 2007 schrieb Nick Piggin:
>> > On Monday 15 October 2007 18:28, Christian Borntraeger wrote:
>> > > Andrew, this is a resend of a bugfix patch. Ramdisk seems a bit
>> > > unmaintained, so decided to sent the patch to you :-).
>> > > I have CCed Ted, who did work on the code in the 90s. I found no
>> > > current email address of Chad Page.
>> >
>> > This really needs to be fixed...
>>
>> I obviously agree ;-)
>> We have seen this problem happen several times.
>>
>> > I can't make up my mind between the approaches to fixing it.
>> >
>> > On one hand, I would actually prefer to really mark the buffers
>> > dirty (as in: Eric's fix for this problem[*]) than this patch,
>> > and this seems a bit like a bandaid...
>>
>> I have never seen these patches, so I cannot comment on them.
>
>> > On the other hand, the wound being covered by the bandaid is
>> > actually the code in the buffer layer that does this latent
>> > "cleaning" of the page because it sadly doesn't really keep
>> > track of the pagecache state. But it *still* feels like we
>> > should be marking the rd page's buffers dirty which should
>> > avoid this problem anyway.
>>
>> Yes, that would solve the problem as well. As long as we fix
>> the problem, I am happy. On the other hand, do you see any
>> obvious problem with this "bandaid"?
>
> I don't think so -- in fact, it could be the best candidate for
> a minimal fix for stable kernels (anyone disagree? if not, maybe
> you could also send this to the stable maintainers?).

A minor one.  It still leaves us with buffer heads out of sync with
struct page.

> But I do want to have this fixed in a "nice" way. eg. I'd like
> it to mark the buffers dirty because that actually results in
> more reuse of generic kernel code, and also should make rd
> behave more naturally (I like using it to test filesystems
> because it can expose a lot more concurrency than something like
> loop on tmpfs). It should also be possible to actually have
> rd's buffer heads get reclaimed as well, preferably while
> exercising the common buffer paths and without writing much new
> code.

We actually allow that currently for clean pages which is part
of what makes this tricky.

> All of that is secondary to fixing the data corruption problem
> of course! But the fact that those alternate patches do exist now
> means I want to just bring them into the discussion again before
> merging one or the other.

The core of my original fix was to modify init_page_buffers so that
when we added buffers to a dirty page the buffers became dirty.

Modifying the generic code is a bit spooky because it requires us
to audit the kernel to make certain nothing else depends on the
current behavior in odd ways.  Although since init_page_buffers
is only called when we are adding buffer heads to an existing
page I still think that was the proper change.

The historical reason for my patches not getting merged the first
time is there was some weird issue with reiserfs ramdisks and so
Andrew disabled the code, and then dropped it when he had discovered
he had the patch disabled for several releases.  I don't think
any causal relationship was ever established.  But I didn't
hear enough about the reiserfs ramdisk issue, to make a guess
what was going on.

So it looks to me like the important invariant we need to maintain
is that when a ramdisk page is dirty it always has buffers and those
buffers are dirty as well.  With a little care we can ensure this
happens with just modifications to rd.c

Eric
















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