linux-ext4 - Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode eviction path

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Date:   Thu, 22 Sep 2022 17:41:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Chinner <david@...morbit.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Dan Williams <dan.j.williams@...el.com>,
        <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>,
        Jan Kara <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        <linux-fsdevel@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
        <linux-xfs@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH v2 05/18] xfs: Add xfs_break_layouts() to the inode
 eviction path

Dave Chinner wrote:
> On Wed, Sep 21, 2022 at 07:28:51PM -0300, Jason Gunthorpe wrote:
> > On Thu, Sep 22, 2022 at 08:14:16AM +1000, Dave Chinner wrote:
> > 
> > > Where are these DAX page pins that don't require the pin holder to
> > > also hold active references to the filesystem objects coming from?
> > 
> > O_DIRECT and things like it.
> 
> O_DIRECT IO to a file holds a reference to a struct file which holds
> an active reference to the struct inode. Hence you can't reclaim an
> inode while an O_DIRECT IO is in progress to it. 
> 
> Similarly, file-backed pages pinned from user vmas have the inode
> pinned by the VMA having a reference to the struct file passed to
> them when they are instantiated. Hence anything using mmap() to pin
> file-backed pages (i.e. applications using FSDAX access from
> userspace) should also have a reference to the inode that prevents
> the inode from being reclaimed.
> 
> So I'm at a loss to understand what "things like it" might actually
> mean. Can you actually describe a situation where we actually permit
> (even temporarily) these use-after-free scenarios?

Jason mentioned a scenario here:

https://lore.kernel.org/all/YyuoE8BgImRXVkkO@nvidia.com/

Multi-thread process where thread1 does open(O_DIRECT)+mmap()+read() and
thread2 does memunmap()+close() while the read() is inflight.

Sounds plausible to me, but I have not tried to trigger it with a focus
test.