Re: [PATCH 3/5] mm/slub: Improve redzone check and zeroing for krealloc()

[Feng Tang]

Hi Danilo,

Thanks for the review!

On Tue, Sep 10, 2024 at 12:06:05PM +0200, Danilo Krummrich wrote:
> On Mon, Sep 09, 2024 at 09:29:56AM +0800, Feng Tang wrote:
> > For current krealloc(), one problem is its caller doesn't know what's
> > the actual request size, say the object is 64 bytes kmalloc one, but
> > the original caller may only requested 48 bytes. And when krealloc()
> > shrinks or grows in the same object, or allocate a new bigger object,
> > it lacks this 'original size' information to do accurate data preserving
> > or zeroing (when __GFP_ZERO is set).
> > 
> > And when some slub debug option is enabled, kmalloc caches do have this
> > 'orig_size' feature. So utilize it to do more accurate data handling,
> > as well as enforce the kmalloc-redzone sanity check.
> > 
> > The krealloc() related code is moved from slab_common.c to slub.c for
> > more efficient function calling.
> 
> I think it would be good to do this in a separate commit, for a better diff and
> history.

Agreed. will do.

> > 
> > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > ---
> >  mm/slab_common.c |  84 -------------------------------------
> >  mm/slub.c        | 106 +++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 106 insertions(+), 84 deletions(-)
[...]
> > +
> > +/**
> > + * krealloc - reallocate memory. The contents will remain unchanged.
> > + * @p: object to reallocate memory for.
> > + * @new_size: how many bytes of memory are required.
> > + * @flags: the type of memory to allocate.
> > + *
> > + * If @p is %NULL, krealloc() behaves exactly like kmalloc().  If @new_size
> > + * is 0 and @p is not a %NULL pointer, the object pointed to is freed.
> > + *
> > + * If __GFP_ZERO logic is requested, callers must ensure that, starting with the
> > + * initial memory allocation, every subsequent call to this API for the same
> > + * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
> > + * __GFP_ZERO is not fully honored by this API.
> > + *
> > + * When slub_debug_orig_size() is off,  since krealloc() only knows about the
> 
> I think you want to remove ' since ' here.

Yes! :)

> > + * bucket size of an allocation (but not the exact size it was allocated with)
> > + * and hence implements the following semantics for shrinking and growing
> > + * buffers with __GFP_ZERO.
> > + *
> > + *         new             bucket
> > + * 0       size             size
> > + * |--------|----------------|
> > + * |  keep  |      zero      |
> > + *
> > + * Otherwize, the original allocation size 'orig_size' could be used to
> 
> Typo in 'otherwise'.

Will fix.

Thanks,
Feng