On Tue, Jul 23, 2024 at 09:50:13AM +0200, Michal Hocko wrote: > On Mon 22-07-24 18:29:24, Danilo Krummrich wrote: > > Besides the obvious (and desired) difference between krealloc() and > > kvrealloc(), there is some inconsistency in their function signatures > > and behavior: > > > > - krealloc() frees the memory when the requested size is zero, whereas > > kvrealloc() simply returns a pointer to the existing allocation. > > > > - krealloc() behaves like kmalloc() if a NULL pointer is passed, whereas > > kvrealloc() does not accept a NULL pointer at all and, if passed, > > would fault instead. > > > > - krealloc() is self-contained, whereas kvrealloc() relies on the caller > > to provide the size of the previous allocation. > > > > Inconsistent behavior throughout allocation APIs is error prone, hence make > > kvrealloc() behave like krealloc(), which seems superior in all mentioned > > aspects. > > I completely agree with this. Fortunately the number of existing callers > is small and none of them really seem to depend on the current behavior > in that aspect. > > > Besides that, implementing kvrealloc() by making use of krealloc() and > > vrealloc() provides oppertunities to grow (and shrink) allocations more > > efficiently. For instance, vrealloc() can be optimized to allocate and > > map additional pages to grow the allocation or unmap and free unused > > pages to shrink the allocation. > > This seems like a change that is independent on the above and should be > a patch on its own. The optimizations you mean? Yes, I intend to do this in a separate series. For now, I put TODOs in vrealloc. > > [...] > > > diff --git a/mm/util.c b/mm/util.c > > index bc488f0121a7..0ff5898cc6de 100644 > > --- a/mm/util.c > > +++ b/mm/util.c > > @@ -608,6 +608,28 @@ unsigned long vm_mmap(struct file *file, unsigned long addr, > > } > > EXPORT_SYMBOL(vm_mmap); > > > > +static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size) > > This seems like a generally useful helper which it is not. I would call > it something like __kvmalloc_gfp_adjust or something similar so that it is > clear that this is just a helper to adjust gfp flag for slab allocator > path Christoph proposed this name, I think he wanted to encode the target of the flags, whereas you want to encode where the function is intended to be called from. When I originally named this thing, I had the same conflict - encoding both turns out clumsy - and came up with to_kmalloc_flags(). Personally, I'd be fine with __kvmalloc_gfp_adjust() too. > > [...] > > -void *kvrealloc_noprof(const void *p, size_t oldsize, size_t newsize, gfp_t flags) > > +/** > > + * kvrealloc - reallocate 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). > > + * > > + * If @p is %NULL, kvrealloc() behaves exactly like kvmalloc(). If @size is 0 > > + * and @p is not a %NULL pointer, the object pointed to is freed. > > + * > > + * Return: pointer to the allocated memory or %NULL in case of error > > + */ > > +void *kvrealloc_noprof(const void *p, size_t size, gfp_t flags) > > { > > - void *newp; > > + void *n; > > + > > if (!size && p) { > kvfree(p); > return NULL; > } > > would make this code flow slightly easier to read because the freeing > path would be shared for all compbinations IMO. Personally, I like it without. For me the simplicity comes from directing things to either krealloc() or vrealloc(). But I'd be open to change it however. > > > + if (is_vmalloc_addr(p)) > > + return vrealloc_noprof(p, size, flags); > > + > > + n = krealloc_noprof(p, size, kmalloc_gfp_adjust(flags, size)); > > + if (!n) { > > + /* We failed to krealloc(), fall back to kvmalloc(). */ > > + n = kvmalloc_noprof(size, flags); > > Why don't you simply use vrealloc_noprof here? We could do that, but we'd also need to do the same checks kvmalloc() does, i.e. /* * It doesn't really make sense to fallback to vmalloc for sub page * requests */ if (ret || size <= PAGE_SIZE) return ret; /* non-sleeping allocations are not supported by vmalloc */ if (!gfpflags_allow_blocking(flags)) return NULL; /* Don't even allow crazy sizes */ if (unlikely(size > INT_MAX)) { WARN_ON_ONCE(!(flags & __GFP_NOWARN)); return NULL; } Does the kmalloc() retry through kvmalloc() hurt us enough to do that? This should only ever happen when we switch from a kmalloc buffer to a vmalloc buffer, which we only do once, we never switch back. > > > + if (!n) > > + return NULL; > > + > > + if (p) { > > + /* We already know that `p` is not a vmalloc address. */ > > + memcpy(n, p, ksize(p)); > > + kfree(p); > > + } > > + } > > > > - if (oldsize >= newsize) > > - return (void *)p; > > - newp = kvmalloc_noprof(newsize, flags); > > - if (!newp) > > - return NULL; > > - memcpy(newp, p, oldsize); > > - kvfree(p); > > - return newp; > > + return n; > > } > > EXPORT_SYMBOL(kvrealloc_noprof); > > > > -- > > 2.45.2 > > -- > Michal Hocko > SUSE Labs >