On 24.04.24 22:31, Vincent Donnefort wrote:
Hi David,
Thanks for your quick response.
On Wed, Apr 24, 2024 at 05:26:39PM +0200, David Hildenbrand wrote:
I gave it some more thought, and I think we are still missing something (I
wish PFNMAP/MIXEDMAP wouldn't be that hard).
+
+/*
+ * +--------------+ pgoff == 0
+ * | meta page |
+ * +--------------+ pgoff == 1
+ * | subbuffer 0 |
+ * | |
+ * +--------------+ pgoff == (1 + (1 << subbuf_order))
+ * | subbuffer 1 |
+ * | |
+ * ...
+ */
+#ifdef CONFIG_MMU
+static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
+ struct vm_area_struct *vma)
+{
+ unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
+ unsigned int subbuf_pages, subbuf_order;
+ struct page **pages;
+ int p = 0, s = 0;
+ int err;
+
I'd add some comments here like
/* Refuse any MAP_PRIVATE or writable mappings. */
+ if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
+ !(vma->vm_flags & VM_MAYSHARE))
+ return -EPERM;
+
/*
* Make sure the mapping cannot become writable later. Also, tell the VM
* to not touch these pages pages (VM_DONTCOPY | VM_DONTDUMP) and tell
* GUP to leave them alone as well (VM_IO).
*/
+ vm_flags_mod(vma,
+ VM_MIXEDMAP | VM_PFNMAP |
+ VM_DONTCOPY | VM_DONTDUMP | VM_DONTEXPAND | VM_IO,
+ VM_MAYWRITE);
I am still really unsure about VM_PFNMAP ... it's not a PFNMAP at all and,
as stated, vm_insert_pages() even complains quite a lot when it would have
to set VM_MIXEDMAP and VM_PFNMAP is already set, likely for a very good
reason.
Can't we limit ourselves to VM_IO?
But then, I wonder if it really helps much regarding GUP: yes, it blocks
ordinary GUP (see check_vma_flags()) but as insert_page_into_pte_locked()
does *not* set pte_special(), GUP-fast (gup_fast_pte_range()) will not
reject it.
Really, if you want GUP-fast to reject it, remap_pfn_range() and friends are
the way to go, that will set pte_special() such that also GUP-fast will
leave it alone, just like vm_normal_page() would.
So ... I know Linus recommended VM_PFNMAP/VM_IO to stop GUP, but it alone
won't stop all of GUP. We really have to mark the PTE as special, which
vm_insert_page() must not do (because it is refcounted!).
Hum, apologies, I am not sure to follow the connection here. Why do you think
the recommendation was to prevent GUP?
Ah, I'm hallucinating! :) "not let people play games with the mapping" to me
implied "make sure nobody touches it". If GUP is acceptable that makes stuff
a lot easier. VM_IO will block some GUP, but not all of it.
Which means: do we really have to stop GUP from grabbing that page?
Using vm_insert_page() only with VM_MIXEDMAP (and without VM_PFNMAP|VM_IO)
would be better.
Under the assumption we do not want to stop all GUP, why not using VM_IO over
VM_MIXEDMAP which is I believe more restrictive?
VM_MIXEDMAP will be implicitly set by vm_insert_page(). There is a lengthy comment
for vm_normal_page() that explains all this madness. VM_MIXEDMAP is primarily
relevant for COW mappings, which you just forbid completely.
remap_pfn_range_notrack() documents the semantics of some of the other flags:
* VM_IO tells people not to look at these pages
* (accesses can have side effects).
* VM_PFNMAP tells the core MM that the base pages are just
* raw PFN mappings, and do not have a "struct page" associated
* with them.
* VM_DONTEXPAND
* Disable vma merging and expanding with mremap().
* VM_DONTDUMP
* Omit vma from core dump, even when VM_IO turned off.
VM_PFNMAP is very likely really not what we want, unless we really perform raw
PFN mappings ... VM_IO we can set without doing much harm.
So I would suggest dropping VM_PFNMAP when using vm_insert_pages(), using only VM_IO
and likely just letting vm_insert_pages() set VM_MIXEDMAP for you.
[...]
vm_insert_pages() documents: "In case of error, we may have mapped a subset
of the provided pages. It is the caller's responsibility to account for this
case."
Which could for example happen, when allocating a page table fails.
Would we able to deal with that here?
As we are in the mmap path, on an error, I would expect the vma to be destroyed
and those pages whom insertion succeeded to be unmapped?
Ah, we simply fail ->mmap().
In mmap_region(), if call_mmap() failed, we "goto unmap_and_free_vma" where we have
/* Undo any partial mapping done by a device driver. */
unmap_region(mm, &vmi.mas, vma, prev, next, vma->vm_start, vma->vm_end, vma->vm_end, true);
But perhaps shall we proactively zap_page_range_single()?
No mmap_region() should indeed be handling it correctly already!
--
Cheers,
David / dhildenb
