BH_Req question

BH_Req question

[Rajagopal Ananthanarayanan]

Hi,

It seems BH_Req is set on a buffer_head by submit_bh.
What part of the code unsets this flag during normal
operations? One path seems to be block_flushpage->unmap_buffer
->clear_bit(BH_Req), but IIRC block_flushpage is used only
for truncates. There must be another path to unset BH_Req
under normal memory pressure, or (more unambiguously) on IO completion.

So: in what ways can BH_Req be unset?

Thanks for any input, i've been staring at the code for long without avail ...

cheers,

ananth.

PS: In case why the question: I've got a system with tons of
pages with buffers marked BH_Req, so try_to_free_buffers() bails
out thinking that the buffer is busy ...

-- 
--------------------------------------------------------------------------
Rajagopal Ananthanarayanan ("ananth")
Member Technical Staff, SGI.
--------------------------------------------------------------------------

Re: BH_Req question

[Andrea Arcangeli]

On Tue, Apr 10, 2001 at 01:12:02PM -0700, Rajagopal Ananthanarayanan wrote:
> 
> Hi,
> 
> It seems BH_Req is set on a buffer_head by submit_bh.
> What part of the code unsets this flag during normal
> operations? One path seems to be block_flushpage->unmap_buffer
> ->clear_bit(BH_Req), but IIRC block_flushpage is used only
> for truncates. There must be another path to unset BH_Req
> under normal memory pressure, or (more unambiguously) on IO completion.
> 
> So: in what ways can BH_Req be unset?

BH_Req is never unset until the buffer is destroyed (put back on the freelist).
BH_Req only says if such a buffer ever did any I/O yet or not. It is basically
only used to deal with I/O errors in sync_buffers().

> PS: In case why the question: I've got a system with tons of
> pages with buffers marked BH_Req, so try_to_free_buffers() bails
> out thinking that the buffer is busy ...

Either your debugging is wrong or you broke try_to_free_buffers because a
buffer with BH_Req must still be perfectly freeable.

Andrea

Re: BH_Req question

[Rajagopal Ananthanarayanan]

Andrea Arcangeli wrote:
> 
	[ ... ]
> 
> BH_Req is never unset until the buffer is destroyed (put back on the freelist).
> BH_Req only says if such a buffer ever did any I/O yet or not. It is basically
> only used to deal with I/O errors in sync_buffers().

Interesting. Thanks for the explanation. Since submit_bh was setting BH_Req,
I was misled into thinking that end_io would unset it ...


> 
> > PS: In case why the question: I've got a system with tons of
> > pages with buffers marked BH_Req, so try_to_free_buffers() bails
> > out thinking that the buffer is busy ...
> 
> Either your debugging is wrong or you broke try_to_free_buffers because a
> buffer with BH_Req must still be perfectly freeable.


Okay, I got distracted by BH_Req, which I mistook to be in BUFFER_BUSY_BITS.
There was also BH_Lock set on the buffers, which would qualify for BUFFER_BUSY_BITS ...
so may be it is a buffer_locking problem somewhere.

cheers,

ananth.

--------------------------------------------------------------------------
Rajagopal Ananthanarayanan ("ananth")
Member Technical Staff, SGI.
--------------------------------------------------------------------------