linux-kernel - Re: [PATCH] vmalloc: add warning in _

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Date:	Thu, 3 May 2012 16:30:56 +1000
From:	Nick Piggin <npiggin@...il.com>
To:	Sage Weil <sage@...dream.net>
Cc:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kosaki.motohiro@...il.com,
	rientjes@...gle.com, Neil Brown <neilb@...e.de>,
	Artem Bityutskiy <dedekind1@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	James Morris <jmorris@...ei.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] vmalloc: add warning in __vmalloc

On 3 May 2012 15:46, Sage Weil <sage@...dream.net> wrote:
> On Thu, 3 May 2012, Minchan Kim wrote:
>> On 05/03/2012 04:46 AM, Andrew Morton wrote:
>> > Well.  What are we actually doing here?  Causing the kernel to spew a
>> > warning due to known-buggy callsites, so that users will report the
>> > warnings, eventually goading maintainers into fixing their stuff.
>> >
>> > This isn't very efficient :(
>>
>>
>> Yes. I hope maintainers fix it before merging this.
>>
>> >
>> > It would be better to fix that stuff first, then add the warning to
>> > prevent reoccurrences.  Yes, maintainers are very naughty and probably
>> > do need cattle prods^W^W warnings to motivate them to fix stuff, but we
>> > should first make an effort to get these things fixed without
>> > irritating and alarming our users.
>> >
>> > Where are these offending callsites?
>
> Okay, maybe this is a stupid question, but: if an fs can't call vmalloc
> with GFP_NOFS without risking deadlock, calling with GFP_KERNEL instead
> doesn't fix anything (besides being more honest).  This really means that
> vmalloc is effectively off-limits for file systems in any
> writeback-related path, right?

Anywhere it cannot reenter the filesystem, yes. GFP_NOFS is effectively
GFP_KERNEL when calling vmalloc.

Note that in writeback paths, a "good citizen" filesystem should not require
any allocations, or at least it should be able to tolerate allocation failures.
So fixing that would be a good idea anyway.
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