Message-ID: <871vekhigl.fsf@openvz.org>
Date:	Mon, 12 Apr 2010 18:08:10 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Eric Sandeen <sandeen@...hat.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 0/3] ext4: don't use quota reservation for speculative metadata blocks

Jan Kara <jack@...e.cz> writes:

>> Jan Kara <jack@...e.cz> writes:
>> 
>> >> Eric Sandeen <sandeen@...hat.com> writes:
>> >> 
>> >> > Dmitry Monakhov wrote:
>> >> >> Eric Sandeen <sandeen@...hat.com> writes:
>> >> >> 
>> >> >>> Because we can badly over-reserve metadata when we
>> >> >>> calculate worst-case, it complicates things for quota, since
>> >> >>> we must reserve and then claim later, retry on EDQUOT, etc.
>> >> >>> Quota is also a generally smaller pool than fs free blocks,
>> >> >>> so this over-reservation hurts more, and more often.
>> >> >>>
>> >> >>> I'm of the opinion that it's not the worst thing to allow
>> >> >>> metadata to push a user slightly over quota.  This simplifies
>> >> >>> the code and avoids the false quota rejections that result
>> >> >>> from worst-case speculation.
>> >> >> Hm.. Totally agree with issue description. And seem there is no another
>> >> >> solution except yours.
>> >> >> ASAIU alloc_nofail is called from places where it is impossible to fail
>> >> >> an allocation even if something goes wrong.
>> >> >> I ask because currently i'm working on EIO handling in alloc/free calls.
>> >> >> I've found that it is useless to fail claim/free procedures because
>> >> >> caller is unable to handle it properly.
>> >> >> It is impossible to fail following operation
>> >> >> ->writepage
>> >> >>  ->dquot_claim_space (what to do if EIO happens?)
>> >> >
>> >> > Hm, if these start returning EIO then maybe my patch should be modified
>> >> > to treat EDQUOT differently than EIO ... assuming callers can handle
>> >> > the return at all.
>> >> >
>> >> > In other words, make NOFAIL really just mean "don't fail for EDQUOT"
>> >> Yes. agree So we have two types of errors
>> >> 1) expected errors: EDQUOT
>> >> 2) fatal errors: (EIO/ENOSPC/ENOMEM)
>> >> So we need two types of flags:
>> >> 1)FORCE (IMHO it is better name than you proposed) to allow exceed a
>> >>   quota limit
>> >> 2)NOFAIL to allow ignore fatal errors.
>> >> 
>> >> We still need NOFAIL, because for example if something is happens in
>> >> ->write_page()
>> >>  ->dquot_claim()
>> >>      update_quota() -> EIO  /* update disk quota */
>> >>      update_bytes() /* update i_bytes count */
>> >> It is obvious that write_page should fail because it is too late to
>> >> return the error to userspace, so data will probably lost which
>> >> is much more dramatic bug than quota inconsistency.
>> >> So the only options we have is to:
>> >> 1) Do not modify inode->i_bytes and return error which caller will
>> >>    probably ignore. IMHO this is not good because result in
>> >>    incorrect stat()
>> >> 
>> >> 2) do as much as we can (as it happens for now), modify inode->i_bytes
>> >>    and return positive error code to caller.(which signal what error
>> >>    result in quota inconsystency only)
>> >   Yes, agreed that 2) is a better solution.
>> >
>> >> This fatal errors handling logic i'll post on top of your patch-set.
>> >> But please change flag name from NOFAIL to FORCE.
>> >   Hmm, do we really need to distinguish between your NOFAIL and FORCE?
>> > I mean there are places where we can handle quota failures (both EDQUOT
>> > or others) and places where we cannot and then we just want to go on as
>> > seamlessly as possible. So NOFAIL flag seems to be enough...
>> >   Now I agree that in theory there can be some caller which might wish
>> > to seamlessly continue on EDQUOT and bail out on EIO but I'm not aware
>> > of such callsite currently so there's no immediate need for the flag.
>> > So Eric's patches seem to be fine to me as they are. What do you think?
>> FORCE but without NOFAIL will be useful for example in
>> write to fallocated area
>> extent split/convert (uninitialized=>initialized conversion)
>> It is not good idea to return EDQUOT from write to reserved area
>> due to metadata overhead, but it is easy to handle EIO from that method.
>> So IMHO two flags is not a fancy option, but reasonable design solution.
>> This design confirms to right for openvz's userbean counters.
>> The only thing i ask is to rename NOFAIL => FORCE.
>   But as I wrote in my other email that callsite in ext4 really needs NOFAIL,
> not only FORCE as far as I can see. So Eric's patch is actually right, isn't
> it?
Ohh. Seems we are talking about the same thing but my explanation is
not clear (sorry for my crappy English)
The whole patch is definitely right. All places changed has NOFAIL
(it must succeed in 100% of cases)semantics. And it is reasonable to
accept it as is.
Later i'll add new FORCE flag and:
1) convert all NOFAIL callers to (FORCE|NOFAIL)
2) add new callers of FORCE.
>
>> BTW I'm too familiar with cross-devel-tree process
>> If tytso@ will get the patchset will you get an quota-related patches
>> to linux-fs tree too? Otherwise everybody have to wait for ext4-tree
>> push to linus's tree and when to linux-fs.
>   Yes, I can carry them in my tree as well (I think git is clever enough to
> recognize identical patches in two different trees and do not conflict).
> Only I can push the changes dependent on them only after Ted pushes his
> tree to Linus.
>
> 								Honza
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