Re: [PATCH] mm: Improve comment before pagecache_isize_extended()

["Jan Beulich" <JBeulich@suse.com>]

>>> On 04.11.14 at 16:33, <jack@suse.cz> wrote:
> On Tue 04-11-14 12:20:26, Jan Beulich wrote:
>> >>> On 04.11.14 at 12:43, <"jack@suse.cz".non-mime.internet> wrote:
>> > --- a/mm/truncate.c
>> > +++ b/mm/truncate.c
>> > @@ -743,10 +743,13 @@ EXPORT_SYMBOL(truncate_setsize);
>> >   * changed.
>> >   *
>> >   * The function must be called after i_size is updated so that page fault
>> > - * coming after we unlock the page will already see the new i_size.
>> > - * The function must be called while we still hold i_mutex - this not only
>> > - * makes sure i_size is stable but also that userspace cannot observe new
>> > - * i_size value before we are prepared to store mmap writes at new inode 
> size.
>> > + * coming after we unlock the page will already see the new i_size.  The 
> caller
>> > + * must make sure (generally by holding i_mutex but e.g. XFS uses its 
> private
>> > + * lock) i_size cannot change from the new value while we are called. It 
> must
>> > + * also make sure userspace cannot observe new i_size value before we are
>> > + * prepared to store mmap writes upto new inode size (otherwise userspace 
> could
>> > + * think it stored data via mmap within i_size but they would get zeroed 
> due to
>> > + * writeback & reclaim because they have no backing blocks).
>> >   */
>> >  void pagecache_isize_extended(struct inode *inode, loff_t from, loff_t to)
>> >  {
>> 
>> May I suggest that the comment preceding truncate_setsize() also be
>> updated/removed?
>   But that comment is actually still true AFAICT because VFS takes i_mutex
> before calling into ->setattr(). So we hold i_mutex in truncate_setsize()
> even for XFS.

I doubt that, especially in the light of the WARN_ON() that
prompted all this:

[<ffffffff810053fa>] dump_trace+0x7a/0x350
[<ffffffff810050de>] show_stack_log_lvl+0xee/0x150
[<ffffffff810064fc>] show_stack+0x1c/0x50
[<ffffffff8138e4e3>] dump_stack+0x68/0x7d
[<ffffffff81042c82>] warn_slowpath_common+0x82/0xb0
[<ffffffff810d3831>] pagecache_isize_extended+0x121/0x130
[<ffffffff810d4689>] truncate_setsize+0x29/0x50
[<ffffffffa056705f>] xfs_setattr_size+0x12f/0x440 [xfs]
[<ffffffffa055cbf7>] xfs_file_fallocate+0x297/0x310 [xfs]
[<ffffffff81111b59>] do_fallocate+0x169/0x190
[<ffffffff8111206e>] SyS_fallocate+0x4e/0x90
[<ffffffff81392712>] system_call_fastpath+0x12/0x17
[<00007f0e6bdddf45>] 0x7f0e6bdddf45

I.e. truncate_setsize() is being called here without the mutex
held (or else the WARN_ON() wouldn't have got triggered in
the first place).

Jan

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