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

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Date:   Thu, 27 Apr 2023 18:26:11 -0700
From:   Jane Chu <jane.chu@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
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 4/27/2023 4:48 PM, Matthew Wilcox wrote:
> 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.

Got it, I'll add errno conversion in the respin.

thanks,
-jane