linux-kernel - Re: [PATCH v2] dax: enable dax fault handler to report VM_FAULT

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Message-ID: <ZEsJxwbIeq3jHDBT@casper.infradead.org>
Date:   Fri, 28 Apr 2023 00:48:23 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, vishal.l.verma@...el.com,
        dave.jiang@...el.com, ira.weiny@...el.com, viro@...iv.linux.org.uk,
        brauner@...nel.org, nvdimm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] dax: enable dax fault handler to report
 VM_FAULT_HWPOISON

On Thu, Apr 27, 2023 at 04:36:58PM -0700, Jane Chu wrote:
> > This change results in EHWPOISON leaking to usersapce in the case of
> > read(2), that's not a return code that block I/O applications have ever
> > had to contend with before. Just as badblocks cause EIO to be returned,
> > so should poisoned cachelines for pmem.
> 
> The read(2) man page (https://man.archlinux.org/man/read.2) says
> "On error, -1 is returned, and errno is set to indicate the error. In this
> case, it is left unspecified whether the file position (if any) changes."
> 
> If read(2) users haven't dealt with EHWPOISON before, they may discover that
> with pmem backed dax file, it's possible.

I don't think they should.  While syscalls are allowed to return errnos
other than the ones listed in POSIX, I don't think this is a worthwhile
difference.  We should be abstracting from the user that this is pmem
rather than spinning rust or nand.  So we should convert the EHWPOISON
to EIO as Dan suggests.