linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: bio

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Message-ID: <20191030151444.GC16197@mit.edu>
Date:   Wed, 30 Oct 2019 11:14:45 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     Chao Yu <yuchao0@...wei.com>, Jaegeuk Kim <jaegeuk@...nel.org>,
        Chao Yu <chao@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [f2fs-dev] [PATCH] f2fs: bio_alloc should never fail

On Wed, Oct 30, 2019 at 06:43:45PM +0800, Gao Xiang wrote:
> > You're right, in low memory scenario, allocation with bioset will be faster, as
> > you mentioned offline, maybe we can add/use a priviate bioset like btrfs did
> > rather than using global one, however, we'd better check how deadlock happen
> > with a bioset mempool first ...
> 
> Okay, hope to get hints from Jaegeuk and redo this patch then...

It's not at all clear to me that using a private bioset is a good idea
for f2fs.  That just means you're allocating a separate chunk of
memory just for f2fs, as opposed to using the global pool.  That's an
additional chunk of non-swapable kernel memory that's not going to be
available, in *addition* to the global mempool.  

Also, who else would you be contending for space with the global
mempool?  It's not like an mobile handset is going to have other users
of the global bio mempool.

On a low-end mobile handset, memory is at a premium, so wasting memory
to no good effect isn't going to be a great idea.

Regards,

						- Ted