Re: [PATCH 0/21 v4] dax: Clear dirty bits after flushing caches

[Jan Kara <jack@suse.cz>]

Hi,

forgot to add Kirill to CC since this modifies the fault path he changed
recently. I don't want to resend the whole series just because of this so
at least I'm pinging him like this...

								Honza
On Tue 01-11-16 23:36:06, Jan Kara wrote:
> Hello,
> 
> this is the fourth revision of my patches to clear dirty bits from radix tree
> of DAX inodes when caches for corresponding pfns have been flushed. This patch
> set is significantly larger than the previous version because I'm changing how
> ->fault, ->page_mkwrite, and ->pfn_mkwrite handlers may choose to handle the
> fault so that we don't have to leak details about DAX locking into the generic
> code. In principle, these patches enable handlers to easily update PTEs and do
> other work necessary to finish the fault without duplicating the functionality
> present in the generic code. I'd be really like feedback from mm folks whether
> such changes to fault handling code are fine or what they'd do differently.
> 
> The patches are based on 4.9-rc1 + Ross' DAX PMD page fault series [1] + ext4
> conversion of DAX IO patch to the iomap infrastructure [2]. For testing,
> I've pushed out a tree including all these patches and further DAX fixes
> to:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git dax
> 
> The patches pass testing with xfstests on ext4 and xfs on my end. I'd be
> grateful for review so that we can push these patches for the next merge
> window.
> 
> [1] http://www.spinics.net/lists/linux-mm/msg115247.html
> [2] Posted an hour ago - look for "ext4: Convert ext4 DAX IO to iomap framework"
> 
> Changes since v3:
> * rebased on top of 4.9-rc1 + DAX PMD fault series + ext4 iomap conversion
> * reordered some of the patches
> * killed ->virtual_address field in vm_fault structure as requested by
>   Christoph
> 
> Changes since v2:
> * rebased on top of 4.8-rc8 - this involved dealing with new fault_env
>   structure
> * changed calling convention for fault helpers
> 
> Changes since v1:
> * make sure all PTE updates happen under radix tree entry lock to protect
>   against races between faults & write-protecting code
> * remove information about DAX locking from mm/memory.c
> * smaller updates based on Ross' feedback
> 
> ----
> Background information regarding the motivation:
> 
> Currently we never clear dirty bits in the radix tree of a DAX inode. Thus
> fsync(2) flushes all the dirty pfns again and again. This patches implement
> clearing of the dirty tag in the radix tree so that we issue flush only when
> needed.
> 
> The difficulty with clearing the dirty tag is that we have to protect against
> a concurrent page fault setting the dirty tag and writing new data into the
> page. So we need a lock serializing page fault and clearing of the dirty tag
> and write-protecting PTEs (so that we get another pagefault when pfn is written
> to again and we have to set the dirty tag again).
> 
> The effect of the patch set is easily visible:
> 
> Writing 1 GB of data via mmap, then fsync twice.
> 
> Before this patch set both fsyncs take ~205 ms on my test machine, after the
> patch set the first fsync takes ~283 ms (the additional cost of walking PTEs,
> clearing dirty bits etc. is very noticeable), the second fsync takes below
> 1 us.
> 
> As a bonus, these patches make filesystem freezing for DAX filesystems
> reliable because mappings are now properly writeprotected while freezing the
> fs.
> 								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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