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@xxxxxxxxxx> >>>>>> >>>>>> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> >>>>>> >>>>>>> --- 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.
