linux-kernel - Re: + mm-shmem-use-kmem_cache_zalloc-in-shmem_alloc

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Message-ID: <20201120193246.GA19677@rlk>
Date:   Sat, 21 Nov 2020 03:32:46 +0800
From:   Hui Su <sh_def@....com>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     hughd@...gle.com, mm-commits@...r.kernel.org, sh_def@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: + mm-shmem-use-kmem_cache_zalloc-in-shmem_alloc_inode.patch
 added to -mm tree

On Mon, Nov 16, 2020 at 10:17:20PM -0800, Hugh Dickins wrote:
> On Mon, 16 Nov 2020, akpm@...ux-foundation.org wrote:
> > 
> > The patch titled
> >      Subject: mm/shmem: use kmem_cache_zalloc in shmem_alloc_inode()
> > has been added to the -mm tree.  Its filename is
> >      mm-shmem-use-kmem_cache_zalloc-in-shmem_alloc_inode.patch
> > 
> > This patch should soon appear at
> >     https://ozlabs.org/~akpm/mmots/broken-out/mm-shmem-use-kmem_cache_zalloc-in-shmem_alloc_inode.patch
> > and later at
> >     https://ozlabs.org/~akpm/mmotm/broken-out/mm-shmem-use-kmem_cache_zalloc-in-shmem_alloc_inode.patch
> > 
> > Before you just go and hit "reply", please:
> >    a) Consider who else should be cc'ed
> >    b) Prefer to cc a suitable mailing list as well
> >    c) Ideally: find the original patch on the mailing list and do a
> >       reply-to-all to that, adding suitable additional cc's
> > 
> > *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
> > 
> > The -mm tree is included into linux-next and is updated
> > there every 3-4 working days
> > 
> > ------------------------------------------------------
> > From: Hui Su <sh_def@....com>
> > Subject: mm/shmem: use kmem_cache_zalloc in shmem_alloc_inode()
> 
> Andrew, Stephen, please revert this untested "cleanup" from your
> trees a.s.a.p: it's a disaster for anyone using shmem/tmpfs.
> 
> > 
> > in shmem_get_inode():
> > new_inode();
> >   new_inode_pseudo();
> >     alloc_inode();
> >       ops->alloc_inode(); -> shmem_alloc_inode()
> >         kmem_cache_alloc();
> > 
> > memset(info, 0, (char *)inode - (char *)info);
> > 
> > So use kmem_cache_zalloc() in shmem_alloc_inode(),
> > and remove the memset in shmem_get_inode().
> 
> I could not follow that argument at all.  The shmem_inode_cachep
> uses a constructor, and the memset shown is of only a portion of
> the whole inode.  zeroing the entire inode quickly crashes the
> kernel, after showing errors.
> 
> (If you're lucky enough to have a readable display at that point:
> I did not, but got on better with framebuffer than drm/i915; and
> I wonder if there's a separate bug in that area too, because fixing
> this shmem issue is not enough to get my drm/i915 rc4-mm1 booting.)
> 

Sorry, I thought it was a small change before, so I forgot to test it.
It's SO STUPID. I'm really sorry.

> > 
> > Link: https://lkml.kernel.org/r/20201115174026.GA365412@rlk
> > Signed-off-by: Hui Su <sh_def@....com>
> 
> NAK.  Hui Su, please test your "cleanups" before sending them.
> 

I will remember it, thanks.

I have spent two evenings building a patch automated test platform
using qemu and jenkins.

> I'm sorry for being slow to respond, but the priority appeared
> to be to get Matthew Wilcox's series running reliably, so I had not
> got around to checking the less urgent shmem patches before they
> slipped into mmotm - there may well be more that I want to NAK,
> but this is the dangerous one.
> 
> Thanks,
> Hugh
>