Re: PATCH - bug in vfree

Kanoj Sarcar
1999-02-22T20:37:27Z

On Feb 20, 8:46pm, Neil Booth wrote:
> Subject: PATCH - bug in vfree
>

>
> Quick description:- vfree forgets to subtract the extra cushion page
> from the size of each virtual memory area stored in vmlist when it calls
> vmfree_area_pages. This means that only the vmalloc-requested size is
> allocated by vmalloc_area_pages, but the requested size PLUS the cushion
> page is freed by vmfree_area_pages.
>
> More deeply:- Close inspection of get_vm_area reveals that
> (intentionally?) it does NOT insist there be a cushion page behind a VMA
> that is placed in front of a previously-allocated VMA, it ONLY
> guarantees that a cushion page lies in front of newly-allocated VMAs.
> Thus two VMAs could be immediately adjacent without a cushion page, and
> coupled with the vfree bug means that vfree-ing the first VMA also frees
> the first page of the second VMA, with dire consequences.
>
> I have described this as clearly as I can, I hope it makes sense. Alan,
> this same bug also exists in 2.0.36.
>
> Neil.
>
> [ text/plain ] :
>
> --- linux/mm/vmalloc.c~ Sun Jan 24 19:21:06 1999
> +++ linux/mm/vmalloc.c Sat Feb 20 20:17:11 1999
> @@ -187,7 +187,7 @@
> for (p = &vmlist ; (tmp = *p) ; p = &tmp->next) {
> if (tmp->addr == addr) {
> *p = tmp->next;
> - vmfree_area_pages(VMALLOC_VMADDR(tmp->addr),
tmp->size);
> + vmfree_area_pages(VMALLOC_VMADDR(tmp->addr), tmp->size
- PAGE_SIZE);
> kfree(tmp);
> return;
> }
>-- End of excerpt from Neil Booth

On Feb 20, 9:14pm, Neil Booth wrote:
> Subject: Re: PATCH - bug in vfree
> Neil Booth wrote:
>
> > More deeply:- Close inspection of get_vm_area reveals that
> > (intentionally?) it does NOT insist there be a cushion page behind a VMA
> > that is placed in front of a previously-allocated VMA, it ONLY
> > guarantees that a cushion page lies in front of newly-allocated VMAs.
>
> Sorry, this is not correct (mistook < for <=). The bug report is
> correct, though.
>
> Neil.

Given that we agree that there is always one page between vm_structs,
the extra page freeing (in vfree) is probably inconsequential, given
that vmfree_area_pages/free_area_pmd/free_area_pte basically ignores
null ptes. But yes, I agree it would be nice to get the "bug" fixed
in vfree().

Kanoj