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Re: [PATCH 3/3] Add timeout feature

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/3] Add timeout feature
From: Arjan van de Ven <arjan@xxxxxxxxxxxxx>
Date: Wed, 9 Jul 2008 06:53:36 -0700
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>, hch@xxxxxxxxxxxxx, tytso@xxxxxxx, pavel@xxxxxxx, t-sato@xxxxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, viro@xxxxxxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, dm-devel@xxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, axboe@xxxxxxxxx, mtk.manpages@xxxxxxxxxxxxxx
In-reply-to: <20080709071346.GS11558@disturbed>
Organization: Intel
References: <20080708231026.GP11558@disturbed> <20080708232031.GE18195@xxxxxxxxxx> <20080709005254.GQ11558@disturbed> <20080709010922.GE9957@xxxxxxx> <E1KGSvZ-0006dB-53@xxxxxxxxxxxxxxxxxxx> <20080709061621.GA5260@xxxxxxxxxxxxx> <E1KGT4q-0006fD-Jb@xxxxxxxxxxxxxxxxxxx> <20080708234120.5072111f@xxxxxxxxxxxxx> <E1KGTTm-0006ke-Jh@xxxxxxxxxxxxxxxxxxx> <20080708235502.1c52a586@xxxxxxxxxxxxx> <20080709071346.GS11558@disturbed>
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On Wed, 9 Jul 2008 17:13:46 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> > one can argue about the need of doing the first 3 steps via a
> > userland loop; it sure sounds like one needs to be really careful
> > to not do any writes to the fs from the app that does snapshots
> > (and that includes doing any syscalls in the kernel that allocate
> > memory.. just because that already could cause unrelated data to be
> > written from inside the app. Not fun.)
> 
> Bloody hell! Doesn't *anyone* understand that a frozen filesystem is
> *clean*? That the process of freezing it ensures all dirty data and
> metadata is written out before the freeze completes? And that once
> frozen, it can't be dirtied until unfrozen?
> 
> That's 3 (or is it 4 - maybe 5 now that I think about it) different
> ppl in 24 hours that have made this same broken argument about
> being unable to write back dirty data on a frozen filesystem......

unless you also freeze the system as a whole (in a 'refrigerator
suspend' way).. the "clean" part is just about a nanosecond long. After
that stuff gets dirty again (you're doing that sendfile to receive more
packets from the FTP upload etc etc).
Sure you can pause those. But there's a real risk that you end up
pausing the app that you want to unfreeze the fs (via the memory
allocation->direct reclaim path). And no, mlock doesn't help.
Especially with delayed allocation, where data writes will cause
metadata activity, this is not just theory.

-- 
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