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

[Danilo Krummrich <dakr@kernel.org>]

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 we should indeed just document that __GFP_ZERO doesn't work for
> re-allocating memory and start to warn about it. As already mentioned, I think
> we should at least gurantee that *realloc(NULL, size, flags | __GFP_ZERO) is
> valid, i.e. WARN_ON(p && flags & __GFP_ZERO).

Maybe I spoke a bit to soon with this last paragraph. I think continuously
gowing something with __GFP_ZERO is a legitimate use case. I just did a quick
grep for users of krealloc() with __GFP_ZERO and found 18 matches.

So, I think, at least for now, we should instead document that __GFP_ZERO is
only fully honored when the buffer is grown continuously (without intermediate
shrinking) and __GFP_ZERO is supplied in every iteration.

In case I miss something here, and not even this case is safe, it looks like
we have 18 broken users of krealloc().

> 
> > 
> > nommu would still uses krealloc() though...
> > 
> > > 
> > > But it's also fundamentally not true for krealloc(), or kvrealloc()
> > > switching from a kmalloc to valloc. ksize() returns the size of the kmalloc
> > > bucket, we don't know what was the exact prior allocation size.
> > 
> > Probably a stupid question, but can't we just zero the full bucket initially and
> > make sure to memset() spare memory in the bucket to zero when krealloc() is
> > called with new_size < ksize()?
> > 
> > > Worse, we
> > > started poisoning the padding in debug configurations, so even a
> > > kmalloc(__GFP_ZERO) followed by krealloc(__GFP_ZERO) can give you unexpected
> > > poison now...
> > 
> > As in writing magics directly to the spare memory in the bucket? Which would
> > then also be copied over to a new buffer in __do_krealloc()?
> > 
> > > 
> > > I guess we should just document __GFP_ZERO is not honored at all for
> > > realloc, and maybe start even warning :/ Hopefully nobody relies on that.
> > 
> > I think it'd be great to make __GFP_ZERO work in all cases. However, if that's
> > really not possible, I'd prefer if we could at least gurantee that
> > *realloc(NULL, size, flags | __GFP_ZERO) is a valid call, i.e.
> > WARN_ON(p && flags & __GFP_ZERO).
> > 
> > > 
> > > > + *
> > > > + * If @p is %NULL, vrealloc() behaves exactly like vmalloc(). If @size is 0 and
> > > > + * @p is not a %NULL pointer, the object pointed to is freed.
> > > > + *
> > > > + * Return: pointer to the allocated memory; %NULL if @size is zero or in case of
> > > > + *         failure
> > > > + */
> > > > +void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
> > > > +{
> > > > +	size_t old_size = 0;
> > > > +	void *n;
> > > > +
> > > > +	if (!size) {
> > > > +		vfree(p);
> > > > +		return NULL;
> > > > +	}
> > > > +
> > > > +	if (p) {
> > > > +		struct vm_struct *vm;
> > > > +
> > > > +		vm = find_vm_area(p);
> > > > +		if (unlikely(!vm)) {
> > > > +			WARN(1, "Trying to vrealloc() nonexistent vm area (%p)\n", p);
> > > > +			return NULL;
> > > > +		}
> > > > +
> > > > +		old_size = get_vm_area_size(vm);
> > > > +	}
> > > > +
> > > > +	if (size <= old_size) {
> > > > +		/*
> > > > +		 * TODO: Shrink the vm_area, i.e. unmap and free unused pages.
> > > > +		 * What would be a good heuristic for when to shrink the
> > > > +		 * vm_area?
> > > > +		 */
> > > > +		return (void *)p;
> > > > +	}
> > > > +
> > > > +	/* TODO: Grow the vm_area, i.e. allocate and map additional pages. */
> > > > +	n = __vmalloc_noprof(size, flags);
> > > > +	if (!n)
> > > > +		return NULL;
> > > > +
> > > > +	if (p) {
> > > > +		memcpy(n, p, old_size);
> > > > +		vfree(p);
> > > > +	}
> > > > +
> > > > +	return n;
> > > > +}
> > > > +
> > > >  #if defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA32)
> > > >  #define GFP_VMALLOC32 (GFP_DMA32 | GFP_KERNEL)
> > > >  #elif defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA)
> > >