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Re: page_buf breakage...

To: Daniel Moore <dxm@xxxxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: page_buf breakage...
From: Russell Cattelan <cattelan@xxxxxxxxxxx>
Date: Thu, 13 Jul 2000 00:30:58 -0500
Cc: linux-xfs@xxxxxxxxxxx
References: <200007130432.OAA69272@xxxxxxxxxxxxxxxxxxxxxxxx>
Reply-to: cattelan@xxxxxxxxxxx
Sender: owner-linux-xfs@xxxxxxxxxxx
Daniel Moore wrote:

> Russell Cattelan writes:
>  => Yes I would guess so.
>  => Setting the length to the count is not what you want to do.
>  => The buffer_length must remain set to whatever size the page_buf was
>  => set up with.
> I don't think I'm doing that.

  #define XFS_BUF_SET_SIZE(bp, cnt)             \
!                       ((bp)->pb_count_desired = (bp)->pb_buffer_length =

> Currently pb_count_desired either doesn't
> get set (it's often zero)

> or it gets set to some masked version of

It isn't used for user data... for meta data it should set...
from pagebuf_get
 /* For inodes without mapping functions we can always fill in
         * the block number now.
        if (PB_BMAP_FUNC(pb) == NULL) {
                /* Use identity mapping */
                pb->pb_bn = pb->pb_file_offset >> PB_SECTOR_BITS(pb);
                pb->pb_count_desired =
                    pb->pb_buffer_length & ~(PB_SECTOR_SIZE(pb) - 1);

If you put the printk in from pagebuf_segment_apply
you will see the count and lenght values during recovery.

> pb_buffer_length. The changes to xfs_buf.h make sure that
> pb_count_desired gets updated whenever pb_buffer_length does and some
> changes to page_buf.c stop it setting pb_count_desired to the masked value.

>  => The count_desired is set at pagebuf_get time and will remain equal to
>  => length as long as it isn't reset... as in the case of the log write
>  => There is also a possibility that shrinking the length will cause
>  => the page_buf to lose pages.... I'm not sure about I'll have to look.
> I saw some funny stuff when I tried your patch which (from memory)
> set kb->count to pb_count_desired... I suspect that causes loss...

Hmm shouldn't; the kb->count is only used for the io loop, the pages
are a array off the kiovec and should be free'ed all in one go.
Like I said I didn't have much time to test it, I may have missed something.
I look closer.

I think we need to keep the length and the count uniq... if we
can set them to one another then we can get rid of one?!

Tomorrow morning... I need some sleep.

>  => I'll look at the non kiobuf case it shouldn't be to hard to fix.
> That'd be great. thanks...
> -----------------------------------------------------
>  Daniel Moore                  dxm@xxxxxxx
>  R&D Software Engineer         Phone: +61-3-98348209
>  SGI Performance Tools Group   Fax:   +61-3-98132378
> -----------------------------------------------------

Russell Cattelan

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