Message-ID: <a6866a71-dde9-44a2-8b0e-d6d3c4c702f8@asahilina.net>
Date: Tue, 12 Nov 2024 18:49:46 +0900
From: Asahi Lina <lina@...hilina.net>
To: Jan Kara <jack@...e.cz>
Cc: Dan Williams <dan.j.williams@...el.com>,
 Dave Chinner <david@...morbit.com>, Matthew Wilcox <willy@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Sergio Lopez Pascual
 <slp@...hat.com>, linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH] dax: Allow block size > PAGE_SIZE



On 11/8/24 9:16 PM, Jan Kara wrote:
> On Fri 08-11-24 01:09:54, Asahi Lina wrote:
>> On 11/7/24 7:01 PM, Jan Kara wrote:
>>> On Wed 06-11-24 11:59:44, Dan Williams wrote:
>>>> Jan Kara wrote:
>>>> [..]
>>>>>> This WARN still feels like the wrong thing, though. Right now it is the
>>>>>> only thing in DAX code complaining on a page size/block size mismatch
>>>>>> (at least for virtiofs). If this is so important, I feel like there
>>>>>> should be a higher level check elsewhere, like something happening at
>>>>>> mount time or on file open. It should actually cause the operations to
>>>>>> fail cleanly.
>>>>>
>>>>> That's a fair point. Currently filesystems supporting DAX check for this in
>>>>> their mount code because there isn't really a DAX code that would get
>>>>> called during mount and would have enough information to perform the check.
>>>>> I'm not sure adding a new call just for this check makes a lot of sense.
>>>>> But if you have some good place in mind, please tell me.
>>>>
>>>> Is not the reason that dax_writeback_mapping_range() the only thing
>>>> checking ->i_blkbits because 'struct writeback_control' does writeback
>>>> in terms of page-index ranges?
>>>
>>> To be fair, I don't remember why we've put the assertion specifically into
>>> dax_writeback_mapping_range(). But as Dave explained there's much more to
>>> this blocksize == pagesize limitation in DAX than just doing writeback in
>>> terms of page-index ranges. The whole DAX entry tracking in xarray would
>>> have to be modified to properly support other entry sizes than just PTE &
>>> PMD sizes because otherwise the entry locking just doesn't provide the
>>> guarantees that are expected from filesystems (e.g. you could have parallel
>>> modifications happening to a single fs block in pagesize < blocksize case).
>>>
>>>> All other dax entry points are filesystem controlled that know the
>>>> block-to-pfn-to-mapping relationship.
>>>>
>>>> Recall that dax_writeback_mapping_range() is historically for pmem
>>>> persistence guarantees to make sure that applications write through CPU
>>>> cache to media.
>>>
>>> Correct.
>>>
>>>> Presumably there are no cache coherency concerns with fuse and dax
>>>> writes from the guest side are not a risk of being stranded in CPU
>>>> cache. Host side filesystem writeback will take care of them when / if
>>>> the guest triggers a storage device cache flush, not a guest page cache
>>>> writeback.
>>>
>>> I'm not so sure. When you call fsync(2) in the guest on virtiofs file, it
>>> should provide persistency guarantees on the file contents even in case of
>>> *host* power failure. So if the guest is directly mapping host's page cache
>>> pages through virtiofs, filemap_fdatawrite() call in the guest must result
>>> in fsync(2) on the host to persist those pages. And as far as I vaguely
>>> remember that happens by KVM catching the arch_wb_cache_pmem() calls and
>>> issuing fsync(2) on the host. But I could be totally wrong here.
>>
>> I don't think that's how it actually works, at least on arm64.
>> arch_wb_cache_pmem() calls dcache_clean_pop() which is either dc cvap or
>> dc cvac. Those are trapped by HCR_EL2<TPC>, and that is never set by KVM.
>>
>> There was some discussion of this here:
>> https://lore.kernel.org/all/20190702055937.3ffpwph7anvohmxu@US-160370MP2.local/
> 
> I see. Thanks for correcting me.
> 
>> But I'm not sure that all really made sense then.
>>
>> msync() and fsync() should already provide persistence. Those end up
>> calling vfs_fsync_range(), which becomes a FUSE fsync(), which fsyncs
>> (or fdatasyncs) the whole file. What I'm not so sure is whether there
>> are any other codepaths that also need to provide those guarantees which
>> *don't* end up calling fsync on the VFS. For example, the manpages kind
>> of imply munmap() syncs, though as far as I can tell that's not actually
>> the case. If there are missing sync paths, then I think those might just
>> be broken right now...
> 
> munmap(2) is not an issue because that has no persistency guarantees in
> case of power failure attached to it. Thinking about it some more I agree
> that just dropping dax_writeback_mapping_range() from virtiofs should be
> safe. The modifications are going to be persisted by the host eventually
> (so writeback as such isn't needed) and all crash-safe guarantees are
> revolving around calls like fsync(2), sync(2), sync_fs(2) which get passed
> by fuse and hopefully acted upon on the host. I'm quite confident with this
> because even standard filesystems such as ext4 flush disk caches only in
> response to operations like these (plus some in journalling code but that's
> a separate story).
> 
> 								Honza

I think we should go with that then. Should I send it as Suggested-by:
Dan or do you want to send it?

~~ Lina

Powered by blists - more mailing lists