linux-kernel - Re: [question] Panic in dax_writeback

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Date:   Wed, 17 Mar 2021 06:45:13 +0000
From:   "chenjun (AM)" <chenjun102@...wei.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        "Xiangrui (Euler)" <rui.xiang@...wei.com>,
        "lizhe (Y)" <lizhe67@...wei.com>, yangerkun <yangerkun@...wei.com>,
        "zhangyi (F)" <yi.zhang@...wei.com>,
        Joao Martins <joao.m.martins@...cle.com>
Subject: Re: [question] Panic in dax_writeback_one

Thanks for the advices. I will check out that.

在 2021/3/17 12:55, Dan Williams 写道:
> On Tue, Mar 16, 2021 at 8:00 PM chenjun (AM) <chenjun102@...wei.com> wrote:
>>
>> 在 2021/3/12 1:25, Dan Williams 写道:
>>> On Thu, Mar 11, 2021 at 4:20 AM Matthew Wilcox <willy@...radead.org> wrote:
>>>>
>>>> On Thu, Mar 11, 2021 at 07:48:25AM +0000, chenjun (AM) wrote:
>>>>> static int dax_writeback_one(struct xa_state *xas, struct dax_device
>>>>> *dax_dev, struct address_space *mapping, void *entry)
>>>>> ----dax_flush(dax_dev, page_address(pfn_to_page(pfn)), count * PAGE_SIZE);
>>>>> The pfn is returned by the driver. In my case, the pfn does not have
>>>>> struct page. so pfn_to_page(pfn) return a wrong address.
>>>>
>>>> I wasn't involved, but I think the right solution here is simply to
>>>> replace page_address(pfn_to_page(pfn)) with pfn_to_virt(pfn).  I don't
>>>> know why Dan decided to do this in the more complicated way.
>>>
>>> pfn_to_virt() only works for the direct-map. If pages are not mapped I
>>> don't see how pfn_to_virt() is expected to work.
>>>
>>> The real question Chenjun is why are you writing a new simulator of
>>> memory as a block-device vs reusing the pmem driver or brd?
>>>
>>
>> Hi Dan
>>
>> In my case, I do not want to take memory to create the struct page of
>> the memory my driver used.
> 
> There are efforts happening to drastically reduce that overhead. You
> might want to check out Joao's work [1]. I think that direction holds
> more promise than trying to extend FS_DAX_LIMITED.
> 
> [1]: http://lore.kernel.org/r/20201208172901.17384-1-joao.m.martins@oracle.com
> 
>> And, I think this is also a problem for DCSSBLK.
> 
> If I understand correctly DAX replaced XIP for S390. There have not
> been reports about this problem, and I can only guess because XIP
> (eXecute-In-Place) is a read-only use case where dax_writeback_one()
> is never triggered, or S390 just isn't using DCSSBLK anymore. The last
> time I touched FS_DAX_LIMITED the DCSSBLK maintainers offered to just
> delete this driver to get it out of the way.
> 
>>
>> So I want to go back the older way if CONFIG_FS_DAX_LIMITED
>>
>> diff --git a/fs/dax.c b/fs/dax.c
>> index b3d27fd..6395e84 100644
>> --- a/fs/dax.c
>> +++ b/fs/dax.c
>> @@ -867,6 +867,9 @@ static int dax_writeback_one(struct xa_state *xas,
>> struct dax_device *dax_dev,
>>    {
>>          unsigned long pfn, index, count;
>>          long ret = 0;
>> +       void *kaddr;
>> +       pfn_t new_pfn_t;
>> +       pgoff_t pgoff;
>>
>>          /*
>>           * A page got tagged dirty in DAX mapping? Something is seriously
>> @@ -926,7 +929,25 @@ static int dax_writeback_one(struct xa_state *xas,
>> struct dax_device *dax_dev,
>>          index = xas->xa_index & ~(count - 1);
>>
>>          dax_entry_mkclean(mapping, index, pfn);
>> -       dax_flush(dax_dev, page_address(pfn_to_page(pfn)), count * PAGE_SIZE);
>> +
>> +       if (!IS_ENABLED(CONFIG_FS_DAX_LIMITED) || pfn_valid(pfn))
>> +               kaddr = page_address(pfn_to_page(pfn));
>> +       else {
>> +               ret = bdev_dax_pgoff(mapping->host->i_sb->s_bdev, pfn <<
>> PFN_SECTION_SHIFT, count << PAGE_SHIFT, &pgoff);
> 
> This is broken:
> 
>      mapping->host->i_sb->s_bdev
> 
> ...there is no guarantee that the superblock associated with the
> mapping is hosted on the same block device associated with the passed
> in dax_device. See dax_rtdev in xfs_open_devices().
> 


-- 
Regards
Chen Jun