Re: [PATCH] perf: map pages in advance

[David Hildenbrand <david@redhat.com>]

On 29.11.24 14:38, Lorenzo Stoakes wrote:
> On Fri, Nov 29, 2024 at 02:24:24PM +0100, David Hildenbrand wrote:
>> On 29.11.24 14:19, Lorenzo Stoakes wrote:
>>> On Fri, Nov 29, 2024 at 02:12:23PM +0100, David Hildenbrand wrote:
>>>> On 29.11.24 14:02, Lorenzo Stoakes wrote:
>>>>> On Fri, Nov 29, 2024 at 01:59:01PM +0100, David Hildenbrand wrote:
>>>>>> On 29.11.24 13:55, Lorenzo Stoakes wrote:
>>>>>>> On Fri, Nov 29, 2024 at 01:45:42PM +0100, David Hildenbrand wrote:
>>>>>>>> On 29.11.24 13:26, Peter Zijlstra wrote:
>>>>>>>>> On Fri, Nov 29, 2024 at 01:12:57PM +0100, David Hildenbrand wrote:
>>>>>>>>>
>>>>>>>>>> Well, I think we simply will want vm_insert_pages_prot() that stops treating
>>>>>>>>>> these things like folios :) . *likely*  we'd want a distinct memdesc/type.
>>>>>>>>>>
>>>>>>>>>> We could start that work right now by making some user (iouring,
>>>>>>>>>> ring_buffer) set a new page->_type, and checking that in
>>>>>>>>>> vm_insert_pages_prot() + vm_normal_page(). If set, don't touch the refcount
>>>>>>>>>> and the mapcount.
>>>>>>>>>>
>>>>>>>>>> Because then, we can just make all the relevant drivers set the type, refuse
>>>>>>>>>> in vm_insert_pages_prot() anything that doesn't have the type set, and
>>>>>>>>>> refuse in vm_normal_page() any pages with this memdesc.
>>>>>>>>>>
>>>>>>>>>> Maybe we'd have to teach CoW to copy from such pages, maybe not. GUP of
>>>>>>>>>> these things will stop working, I hope that is not a problem.
>>>>>>>>>
>>>>>>>>> Well... perf-tool likes to call write() upon these pages in order to
>>>>>>>>> write out the data from the mmap() into a file.
>>>>>>>
>>>>>>> I'm confused about what you mean, write() using the fd should work fine, how
>>>>>>> would they interact with the mmap? I mean be making a silly mistake here
>>>>>>
>>>>>> write() to file from the mmap()'ed address range to *some* file.
>>>>>>
>>>>>
>>>>> Yeah sorry my brain melted down briefly, for some reason was thinking of read()
>>>>> writing into the buffer...
>>>>>
>>>>>> This will GUP the pages you inserted.
>>>>>>
>>>>>> GUP does not work on PFNMAP.
>>>>>
>>>>> Well it _does_ if struct page **pages is set to NULL :)
>>>>
>>>> Hm? :)
>>>>
>>>> check_vma_flags() unconditionally refuses VM_PFNMAP.
>>>
>>> Ha, funny with my name all over git blame there... ok yup missed this, the
>>> vm_normal_page() == NULL stuff must but for mixed map (and those other weird
>>> cases I think you can get0...
>>>
>>> Well good. Where is write() invoking GUP? I'm kind of surprised it's not just
>>> using uaccess?
>>>
>>> One thing to note is I did run all the perf tests with no issues whatsoever. You
>>> would _think_ this would have come up...
>>>
>>> I'm editing some test code to explicitly write() from the buffer anyway to see.
> 
> I just tested it and write() works fine, it uses uaccess afaict as part of the
> lib/iov_iter.c code:
> 
> generic_perform_write()
> -> copy_folio_from_iter_atomic()
> -> copy_page_from_iter_atomic()
> -> __copy_from_iter()
> -> copy_from_user_iter()
> -> raw_copy_from_user()
> -> copy_user_generic()
> -> [uaccess asm]
> 

Ah yes. O_DIRECT is the problematic bit I suspect, which will use GUP.

Ptrace access and friends should also no longer work on these pages, 
likely that's tolerable.

>>>
>>> If we can't do pfnmap, and we definitely can't do mixedmap (because it's
>>> basically entirely equivalent in every way to just faulting in the pages as
>>> before and requires the same hacks) then I will have to go back to the drawing
>>> board or somehow change the faulting code.
>>>
>>> This really sucks.
>>>
>>> I'm not quite sure I even understand why we don't allow GUP used _just for
>>> pinning_ on VM_PFNMAP when it is -in effect- already pinned on assumption
>>> whatever mapped it will maintain the lifetime.
>>>
>>> What a mess...
>>
>> Because VM_PFNMAP is dangerous as hell. To get a feeling for that (and also why I
>> raised my refcounting comment earlier) just read recent:
>>
>> commit 79a61cc3fc0466ad2b7b89618a6157785f0293b3
>> Author: Linus Torvalds <torvalds@linux-foundation.org>
>> Date:   Wed Sep 11 17:11:23 2024 -0700
>>
>>      mm: avoid leaving partial pfn mappings around in error case
>>      As Jann points out, PFN mappings are special, because unlike normal
>>      memory mappings, there is no lifetime information associated with the
>>      mapping - it is just a raw mapping of PFNs with no reference counting of
>>      a 'struct page'.
>>
> 
> I'm _very_ aware of this, having worked extensively on things around this kind
> of issue recently (was a little bit involved with the above one too :), and
> explicitly zap on error in this patch to ensure we leave no broken code paths.
> 
> I agree it's horrible, but we need to have a way of mapping this memory without
> having to 'trick' the faulting mechanism to behave correctly.

What's completely "surprising" to me is, if there is no page_mkwrite, 
but the VMA is writable, then just map the PTE writable ...

> 
> At least in perf's case, we're safe, because we ref count in open/close of VMA's.
> 
> This is a special case due to the R/W, R/O thing.
> 
> In reference to that - you said in another email about mapping one part as a
> separate R/W VMA and another as a separate R/O VMA, problem is all of the perf
> code is set up with its own reference counting mechanism and it's not allowed to
> split/merge etc., so we'd have to totally rework all of that to make that work
> and correctly refcount things.
> 
> It'd be a big task. I don't think that's a reasonable thing to put effort into
> at this time...
> 
> Also who knows if there's somebody, somewhere who _relies_ somehow on this being
> a single mapping...

The main issue here really is that we allow R/O pages in VM_WRITE VMAs, 
and want to make write faults fail :(

What an absolute mess, yeah, without some more core changes 
vm_insert_pages() cannot be used, unfortunately.

-- 
Cheers,

David / dhildenb