RE: [PATCH v3 01/11] pagemap: Introduce ->memory_failure()

["ruansy.fnst@fujitsu.com" <ruansy.fnst@fujitsu.com>]

> -----Original Message-----
> From: ruansy.fnst@fujitsu.com <ruansy.fnst@fujitsu.com>
> Subject: RE: [PATCH v3 01/11] pagemap: Introduce ->memory_failure()
> > > > > >
> > > > > > After the conversation with Dave I don't see the point of this.
> > > > > > If there is a memory_failure() on a page, why not just call
> > > > > > memory_failure()? That already knows how to find the inode and
> > > > > > the filesystem can be notified from there.
> > > > >
> > > > > We want memory_failure() supports reflinked files.  In this
> > > > > case, we are not able to track multiple files from a page(this
> > > > > broken
> > > > > page) because
> > > > > page->mapping,page->index can only track one file.  Thus, I
> > > > > page->introduce this
> > > > > ->memory_failure() implemented in pmem driver, to call
> > > > > ->->corrupted_range()
> > > > > upper level to upper level, and finally find out files who are
> > > > > using(mmapping) this page.
> > > > >
> > > >
> > > > I know the motivation, but this implementation seems backwards.
> > > > It's already the case that memory_failure() looks up the
> > > > address_space associated with a mapping. From there I would expect
> > > > a new 'struct address_space_operations' op to let the fs handle
> > > > the case when there are multiple address_spaces associated with a given
> file.
> > > >
> > >
> > > Let me think about it.  In this way, we
> > >     1. associate file mapping with dax page in dax page fault;
> >
> > I think this needs to be a new type of association that proxies the
> > representation of the reflink across all involved address_spaces.
> >
> > >     2. iterate files reflinked to notify `kill processes signal` by the
> > >           new address_space_operation;
> > >     3. re-associate to another reflinked file mapping when unmmaping
> > >         (rmap qeury in filesystem to get the another file).
> >
> > Perhaps the proxy object is reference counted per-ref-link. It seems
> > error prone to keep changing the association of the pfn while the reflink is
> in-tact.
> Hi, Dan
> 
> I think my early rfc patchset was implemented in this way:
>  - Create a per-page 'dax-rmap tree' to store each reflinked file's (mapping,
> offset) when causing dax page fault.
>  - Mount this tree on page->zone_device_data which is not used in fsdax, so
> that we can iterate reflinked file mappings in memory_failure() easily.
> In my understanding, the dax-rmap tree is the proxy object you mentioned.  If
> so, I have to say, this method was rejected. Because this will cause huge
> overhead in some case that every dax page have one dax-rmap tree.
> 

Hi, Dan

How do you think about this?  I am still confused.  Could you give me some advice?


--
Thanks,
Ruan Shiyang.

> 
> --
> Thanks,
> Ruan Shiyang.
> >
> > > It did not handle those dax pages are not in use, because their
> > > ->mapping are not associated to any file.  I didn't think it through
> > > until reading your conversation.  Here is my understanding: this
> > > case should be handled by badblock mechanism in pmem driver.  This
> > > badblock mechanism will call
> > > ->corrupted_range() to tell filesystem to repaire the data if possible.
> >
> > There are 2 types of notifications. There are badblocks discovered by
> > the driver (see notify_pmem()) and there are memory_failures()
> > signalled by the CPU machine-check handler, or the platform BIOS. In
> > the case of badblocks that needs to be information considered by the
> > fs block allocator to avoid / try-to-repair badblocks on allocate, and
> > to allow listing damaged files that need repair. The memory_failure()
> > notification needs immediate handling to tear down mappings to that
> > pfn and signal processes that have consumed it with
> > SIGBUS-action-required. Processes that have the poison mapped, but have not
> consumed it receive SIGBUS-action-optional.
> >
> > > So, we split it into two parts.  And dax device and block device
> > > won't be
> > mixed
> > > up again.   Is my understanding right?
> >
> > Right, it's only the filesystem that knows that the block_device and
> > the dax_device alias data at the same logical offset. The requirements
> > for sector error handling and page error handling are separate like
> > block_device_operations and dax_operations.
> >
> > > But the solution above is to solve the hwpoison on one or couple
> > > pages, which happens rarely(I think).  Do the 'pmem remove'
> > > operation
> > cause hwpoison too?
> > > Call memory_failure() so many times?  I havn't understood this yet.
> >
> > I'm working on a patch here to call memory_failure() on a wide range
> > for the surprise remove of a dax_device while a filesystem might be
> > mounted. It won't be efficient, but there is no other way to notify
> > the kernel that it needs to immediately stop referencing a page.
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