Re: [PATCH 3/8] mm/swap: prevent concurrent swapon on the same S_ISBLK blockdev

[Hugh Dickins <hughd@google.com>]

On Mon, 3 Feb 2014, Andrew Morton wrote:
> On Mon, 27 Jan 2014 18:03:04 +0800 Weijie Yang <weijie.yang@samsung.com> wrote:
> 
> > When swapon the same S_ISBLK blockdev concurrent, the allocated two
> > swap_info could hold the same block_device, because claim_swapfile()
> > allow the same holder(here, it is sys_swapon function).
> > 
> > To prevent this situation, This patch adds swap_lock protect to ensure
> > we can find this situation and return -EBUSY for one swapon call.
> > 
> > As for S_ISREG swapfile, claim_swapfile() already prevent this scenario
> > by holding inode->i_mutex.
> > 
> > This patch is just for a rare scenario, aim to correct of code.
> > 
> 
> hm, OK.  Would it be saner to pass a unique `holder' to
> claim_swapfile()?  Say, `p'?
> 
> Truly, I am fed up with silly swapon/swapoff races.  How often does
> anyone call these things?  Let's slap a huge lock around the whole
> thing and be done with it?

That answer makes me sad: we can't be bothered to get it right,
even when Weijie goes to the trouble of presenting a series to do so.
But I sure don't deserve a vote until I've actually looked through it.

Hugh

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