linux-ext4 - Re: [PATCH 26/25] libext2fs: call get_alloc

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Message-ID: <20140912175750.GA10150@birch.djwong.org>
Date:	Fri, 12 Sep 2014 10:57:50 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 26/25] libext2fs: call get_alloc_block hook when
 allocating blocks

On Fri, Sep 12, 2014 at 01:35:51PM -0400, Theodore Ts'o wrote:
> On Thu, Sep 11, 2014 at 03:34:47PM -0700, Darrick J. Wong wrote:
> > > Hmm... I wonder if we can get away with changing ext2fs_new_block2(fs,
> > > goal, bmap, ret_blk) so that if bmap is NULL, we change its behavior
> > > so that (a) it tries to use the get_alloc_block() hook if it is present, and
> > > (b) it will try to load the block bitmap if it is not already loaded,
> > > instead of returning an error.
> > 
> > Quite probably.  I tried to avoid API behavioral change, at least for the
> > inital patch, though I was thinking that a general cleanup was probably in
> > order.
> 
> It turns out that making a behavioral change could very well break
> some callers --- including e2fsck (see e2fsck_get_alloc_block for an
> example of wahy).  What I'm currently thinking about is an API sort of
> like this:

I currently patched that the stupid way -- by temporarily swapping
fs->get_alloc_block with NULL in new_block2() while we call the function
pointer.

(Also by fixing e2fsck.)

> errcode_t ext2fs_alloc_blocks(ext2_filsys fs, blk64_t goal,
> 	  	              unsigned int *num_blocks,
> 			      char *block_buf, int flags, blk64_t *ret)
> 
> ... which can be used to efficiently allocate up to *num_blocks blocks
> at a time, much like the mballoc interface.  I suspect that would be
> useful for a number of different cases, including ext2fs_fallocate and
> mk_hugefiles.c.

Sounds familiar:  http://marc.info/?l=linux-ext4&m=139898612510491&w=2

> What I'm currently wondering about is whether it's worth the interface
> complexity to have something like a "struct ext2fs_allocation_request"
> structure, so we can potentially add more hints that a future
> implementation might use, or whether that's not worth it.
> 
> What do folks think?

I'm not sure changing a struct vs. changing whatever parameters we feed into
that function is all that much different.  I guess you could get around
structure size changes by forcing callers to use a library allocator function.
But OTOH large allocations are probably rare.

--D

> 
> 					- Ted
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