Re: [PATCH 2/2] mm: Add kvmalloc_ab_c and kvzalloc_struct

[Julia Lawall]

On Thu, 8 Mar 2018, Matthew Wilcox wrote:

> On Thu, Mar 08, 2018 at 07:24:47AM +0100, Julia Lawall wrote:
> > On Wed, 7 Mar 2018, Matthew Wilcox wrote:
> > > On Wed, Mar 07, 2018 at 10:18:21PM +0100, Julia Lawall wrote:
> > > > > Otherwise, yes, please. We could build a coccinelle rule for
> > > > > additional replacements...
> > > >
> > > > A potential semantic patch and the changes it generates are attached
> > > > below.  Himanshu Jha helped with its development.  Working on this
> > > > uncovered one bug, where the allocated array is too large, because the
> > > > size provided for it was a structure size, but actually only pointers to
> > > > that structure were to be stored in it.
> > >
> > > This is cool!  Thanks for doing the coccinelle patch!  Diffstat:
> > >
> > >  50 files changed, 81 insertions(+), 124 deletions(-)
> > >
> > > I find that pretty compelling.  I'll repost the kvmalloc_struct patch
> > > imminently.
> >
> > Thanks.  So it's OK to replace kmalloc and kzalloc, even though they
> > didn't previously consider vmalloc and even though kmalloc doesn't zero?
>
> We'll also need to replace the corresponding places where those structs
> are freed with kvfree().  Can coccinelle handle that too?

This would be harder to do 100% reliably.  Coccinelle would have to rely
on the structure name or the structure type, if the free is in a different
function.  But I guess that the type should be mostly reliable, since all
instances of allocations of the same type should be transformed in the
same way.

>
> > There are a few other cases that use GFP_NOFS and GFP_NOWAIT, but I didn't
> > transform those because the comment says that the flags should be
> > GFP_KERNEL based.  Should those be transformed too?
>
> The problem with non-GFP_KERNEL allocations is that vmalloc may have to
> allocate page tables, which is always done with an implicit GFP_KERNEL
> allocation.  There's an intent to get rid of GFP_NOFS, but that's not
> been realised yet (and I'm not sure of our strategy to eliminate it ...
> I'll send a separate email about that).  I'm not sure why anything's
> trying to allocate with GFP_NOWAIT; can you send a list of those places?

drivers/dma/fsl-edma.c:

fsl_desc = kzalloc(sizeof(*fsl_desc) + sizeof(struct fsl_edma_sw_tcd) * sg_len, GFP_NOWAIT);

drivers/dma/st_fdma.c:

fdesc = kzalloc(sizeof(*fdesc) + sizeof(struct st_fdma_sw_node) * sg_len,
GFP_NOWAIT);

drivers/dma/pxa_dma.c:

sw_desc = kzalloc(sizeof(*sw_desc) + nb_hw_desc * sizeof(struct
pxad_desc_hw *), GFP_NOWAIT);

julia