Re: [PATCH v2 1/2] mm: vmalloc: implement vrealloc()

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Jul 30, 2024 at 03:58:25PM +0200, Vlastimil Babka wrote:
> On 7/30/24 3:14 PM, Danilo Krummrich wrote:
> > On Tue, Jul 30, 2024 at 02:15:34PM +0200, Vlastimil Babka wrote:
> >> On 7/30/24 3:35 AM, Danilo Krummrich wrote:
> >>> On Mon, Jul 29, 2024 at 09:08:16PM +0200, Danilo Krummrich wrote:
> >>>> On Fri, Jul 26, 2024 at 10:05:47PM +0200, Danilo Krummrich wrote:
> >>>>> On Fri, Jul 26, 2024 at 04:37:43PM +0200, Vlastimil Babka wrote:
> >>>>>> On 7/22/24 6:29 PM, Danilo Krummrich wrote:
> >>>>>>> Implement vrealloc() analogous to krealloc().
> >>>>>>>
> >>>>>>> Currently, krealloc() requires the caller to pass the size of the
> >>>>>>> previous memory allocation, which, instead, should be self-contained.
> >>>>>>>
> >>>>>>> We attempt to fix this in a subsequent patch which, in order to do so,
> >>>>>>> requires vrealloc().
> >>>>>>>
> >>>>>>> Besides that, we need realloc() functions for kernel allocators in Rust
> >>>>>>> too. With `Vec` or `KVec` respectively, potentially growing (and
> >>>>>>> shrinking) data structures are rather common.
> >>>>>>>
> >>>>>>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> >>>>>>
> >>>>>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> >>>>>>
> >>>>>>> --- a/mm/vmalloc.c
> >>>>>>> +++ b/mm/vmalloc.c
> >>>>>>> @@ -4037,6 +4037,65 @@ void *vzalloc_node_noprof(unsigned long size, int node)
> >>>>>>>  }
> >>>>>>>  EXPORT_SYMBOL(vzalloc_node_noprof);
> >>>>>>>  
> >>>>>>> +/**
> >>>>>>> + * vrealloc - reallocate virtually contiguous memory; contents remain unchanged
> >>>>>>> + * @p: object to reallocate memory for
> >>>>>>> + * @size: the size to reallocate
> >>>>>>> + * @flags: the flags for the page level allocator
> >>>>>>> + *
> >>>>>>> + * The contents of the object pointed to are preserved up to the lesser of the
> >>>>>>> + * new and old size (__GFP_ZERO flag is effectively ignored).
> >>>>>>
> >>>>>> Well, technically not correct as we don't shrink. Get 8 pages, kvrealloc to
> >>>>>> 4 pages, kvrealloc back to 8 and the last 4 are not zeroed. But it's not
> >>>>>> new, kvrealloc() did the same before patch 2/2.
> >>>>>
> >>>>> Taking it (too) literal, it's not wrong. The contents of the object pointed to
> >>>>> are indeed preserved up to the lesser of the new and old size. It's just that
> >>>>> the rest may be "preserved" as well.
> >>>>>
> >>>>> I work on implementing shrink and grow for vrealloc(). In the meantime I think
> >>>>> we could probably just memset() spare memory to zero.
> >>>>
> >>>> Probably, this was a bad idea. Even with shrinking implemented we'd need to
> >>>> memset() potential spare memory of the last page to zero, when new_size <
> >>>> old_size.
> >>>>
> >>>> Analogously, the same would be true for krealloc() buckets. That's probably not
> >>>> worth it.
> >>
> >> I think it could remove unexpected bad surprises with the API so why not
> >> do it.
> > 
> > We'd either need to do it *every* time we shrink an allocation on spec, or we
> > only do it when shrinking with __GFP_ZERO flag set, which might be a bit
> > counter-intuitive.
> 
> I don't think it is that much counterintuitive.
> 
> > If we do it, I'd probably vote for the latter semantics. While it sounds more
> > error prone, it's less wasteful and enough to cover the most common case where
> > the actual *realloc() call is always with the same parameters, but a changing
> > size.
> 
> Yeah. Or with hardening enabled (init_on_alloc) it could be done always.
> 

Ok, sounds good. Will go with that then.