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2749f7ef36
[BUG]
With experimental subpage compression enabled, a simple fsstress can
lead to self deadlock on page 720896:
mkfs.btrfs -f -s 4k $dev > /dev/null
mount $dev -o compress $mnt
$fsstress -p 1 -n 100 -w -d $mnt -v -s 1625511156
[CAUSE]
If we have a file layout looks like below:
0 32K 64K 96K 128K
|//| |///////////////|
4K
Then we run delalloc range for the inode, it will:
- Call find_lock_delalloc_range() with @delalloc_start = 0
Then we got a delalloc range [0, 4K).
This range will be COWed.
- Call find_lock_delalloc_range() again with @delalloc_start = 4K
Since find_lock_delalloc_range() never cares whether the range
is still inside page range [0, 64K), it will return range [64K, 128K).
This range meets the condition for subpage compression, will go
through async COW path.
And async COW path will return @page_started.
But that @page_started is now for range [64K, 128K), not for range
[0, 64K).
- writepage_dellloc() returned 1 for page [0, 64K)
Thus page [0, 64K) will not be unlocked, nor its page dirty status
will be cleared.
Next time when we try to lock page [0, 64K) we will deadlock, as there
is no one to release page [0, 64K).
This problem will never happen for regular page size as one page only
contains one sector. After the first find_lock_delalloc_range() call,
the @delalloc_end will go beyond @page_end no matter if we found a
delalloc range or not
Thus this bug only happens for subpage, as now we need multiple runs to
exhaust the delalloc range of a page.
[FIX]
Fix the problem by ensuring the delalloc range we ran at least started
inside @locked_page.
So that we will never get incorrect @page_started.
And to prevent such problem from happening again:
- Make find_lock_delalloc_range() return false if the found range is
beyond @end value passed in.
Since @end will be utilized now, add an ASSERT() to ensure we pass
correct @end into find_lock_delalloc_range().
This also means, for selftests we needs to populate @end before calling
find_lock_delalloc_range().
- New ASSERT() in find_lock_delalloc_range()
Now we will make sure the @start/@end passed in at least covers part
of the page.
- New ASSERT() in run_delalloc_range()
To make sure the range at least starts inside @locked page.
- Use @delalloc_start as proper cursor, while @delalloc_end is always
reset to @page_end.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.