linux-kernel - Re: [PATCH v2] ceph: invalidate pages when doing DIO in encrypted inodes

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Date:   Thu, 7 Apr 2022 09:17:40 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>,
        Luís Henriques <lhenriques@...e.de>
Cc:     Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ceph: invalidate pages when doing DIO in encrypted
 inodes


On 4/6/22 9:41 PM, Jeff Layton wrote:
> On Wed, 2022-04-06 at 21:10 +0800, Xiubo Li wrote:
>> On 4/6/22 7:48 PM, Jeff Layton wrote:
>>> On Wed, 2022-04-06 at 12:33 +0100, Luís Henriques wrote:
>>>> Xiubo Li <xiubli@...hat.com> writes:
>>>>
>>>>> On 4/6/22 6:57 PM, Luís Henriques wrote:
>>>>>> Xiubo Li <xiubli@...hat.com> writes:
>>>>>>
>>>>>>> On 4/1/22 9:32 PM, Luís Henriques wrote:
>>>>>>>> When doing DIO on an encrypted node, we need to invalidate the page cache in
>>>>>>>> the range being written to, otherwise the cache will include invalid data.
>>>>>>>>
>>>>>>>> Signed-off-by: Luís Henriques <lhenriques@...e.de>
>>>>>>>> ---
>>>>>>>>      fs/ceph/file.c | 11 ++++++++++-
>>>>>>>>      1 file changed, 10 insertions(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> Changes since v1:
>>>>>>>> - Replaced truncate_inode_pages_range() by invalidate_inode_pages2_range
>>>>>>>> - Call fscache_invalidate with FSCACHE_INVAL_DIO_WRITE if we're doing DIO
>>>>>>>>
>>>>>>>> Note: I'm not really sure this last change is required, it doesn't really
>>>>>>>> affect generic/647 result, but seems to be the most correct.
>>>>>>>>
>>>>>>>> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
>>>>>>>> index 5072570c2203..b2743c342305 100644
>>>>>>>> --- a/fs/ceph/file.c
>>>>>>>> +++ b/fs/ceph/file.c
>>>>>>>> @@ -1605,7 +1605,7 @@ ceph_sync_write(struct kiocb *iocb, struct iov_iter *from, loff_t pos,
>>>>>>>>      	if (ret < 0)
>>>>>>>>      		return ret;
>>>>>>>>      -	ceph_fscache_invalidate(inode, false);
>>>>>>>> +	ceph_fscache_invalidate(inode, (iocb->ki_flags & IOCB_DIRECT));
>>>>>>>>      	ret = invalidate_inode_pages2_range(inode->i_mapping,
>>>>>>>>      					    pos >> PAGE_SHIFT,
>>>>>>>>      					    (pos + count - 1) >> PAGE_SHIFT);
>>>>>>> The above has already invalidated the pages, why doesn't it work ?
>>>>>> I suspect the reason is because later on we loop through the number of
>>>>>> pages, call copy_page_from_iter() and then ceph_fscrypt_encrypt_pages().
>>>>> Checked the 'copy_page_from_iter()', it will do the kmap for the pages but will
>>>>> kunmap them again later. And they shouldn't update the i_mapping if I didn't
>>>>> miss something important.
>>>>>
>>>>> For 'ceph_fscrypt_encrypt_pages()' it will encrypt/dencrypt the context inplace,
>>>>> IMO if it needs to map the page and it should also unmap it just like in
>>>>> 'copy_page_from_iter()'.
>>>>>
>>>>> I thought it possibly be when we need to do RMW, it may will update the
>>>>> i_mapping when reading contents, but I checked the code didn't find any
>>>>> place is doing this. So I am wondering where tha page caches come from ? If that
>>>>> page caches really from reading the contents, then we should discard it instead
>>>>> of flushing it back ?
>>>>>
>>>>> BTW, what's the problem without this fixing ? xfstest fails ?
>>>> Yes, generic/647 fails if you run it with test_dummy_encryption.  And I've
>>>> also checked that the RMW code was never executed in this test.
>>>>
>>>> But yeah I have assumed (perhaps wrongly) that the kmap/kunmap could
>>>> change the inode->i_mapping.
>>>>
>>> No, kmap/unmap are all about high memory and 32-bit architectures. Those
>>> functions are usually no-ops on 64-bit arches.
>> Yeah, right.
>>
>> So they do nothing here.
>>
>>>> In my debugging this seemed to be the case
>>>> for the O_DIRECT path.  That's why I added this extra call here.
>>>>
>>> I agree with Xiubo that we really shouldn't need to invalidate multiple
>>> times.
>>>
>>> I guess in this test, we have a DIO write racing with an mmap read
>>> Probably what's happening is either that we can't invalidate the page
>>> because it needs to be cleaned, or the mmap read is racing in just after
>>> the invalidate occurs but before writeback.
>> This sounds a possible case.
>>
>>
>>> In any case, it might be interesting to see whether you're getting
>>> -EBUSY back from the new invalidate_inode_pages2 calls with your patch.
>>>
>> If it's really this case maybe this should be retried some where ?
>>
> Possibly, or we may need to implement ->launder_folio.
>
> Either way, we need to understand what's happening first and then we can
> figure out a solution for it.

Yeah, make sense.