Date: Fri, 31 May 2024 12:06:52 -0700
From: Yang Shi <shy828301@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: kernel test robot <oliver.sang@...el.com>, Peter Xu <peterx@...hat.com>, 
	Jason Gunthorpe <jgg@...dia.com>, Vivek Kasireddy <vivek.kasireddy@...el.com>, 
	Rik van Riel <riel@...riel.com>, oe-lkp@...ts.linux.dev, lkp@...el.com, 
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Christopher Lameter <cl@...ux.com>, linux-mm@...ck.org
Subject: Re: [linus:master] [mm] efa7df3e3b: kernel_BUG_at_include/linux/page_ref.h

On Fri, May 31, 2024 at 11:38 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 31.05.24 20:30, Yang Shi wrote:
> > On Fri, May 31, 2024 at 11:24 AM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 31.05.24 20:13, Yang Shi wrote:
> >>> On Fri, May 31, 2024 at 11:07 AM Yang Shi <shy828301@...il.com> wrote:
> >>>>
> >>>> On Fri, May 31, 2024 at 10:46 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>>
> >>>>> On 31.05.24 18:50, Yang Shi wrote:
> >>>>>> On Fri, May 31, 2024 at 1:24 AM kernel test robot <oliver.sang@...el.com> wrote:
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> Hello,
> >>>>>>>
> >>>>>>> kernel test robot noticed "kernel_BUG_at_include/linux/page_ref.h" on:
> >>>>>>>
> >>>>>>> commit: efa7df3e3bb5da8e6abbe37727417f32a37fba47 ("mm: align larger anonymous mappings on THP boundaries")
> >>>>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> >>>>>>>
> >>>>>>> [test failed on linus/master      e0cce98fe279b64f4a7d81b7f5c3a23d80b92fbc]
> >>>>>>> [test failed on linux-next/master 6dc544b66971c7f9909ff038b62149105272d26a]
> >>>>>>>
> >>>>>>> in testcase: trinity
> >>>>>>> version: trinity-x86_64-6a17c218-1_20240527
> >>>>>>> with following parameters:
> >>>>>>>
> >>>>>>>            runtime: 300s
> >>>>>>>            group: group-00
> >>>>>>>            nr_groups: 5
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> compiler: gcc-13
> >>>>>>> test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
> >>>>>>>
> >>>>>>> (please refer to attached dmesg/kmsg for entire log/backtrace)
> >>>>>>>
> >>>>>>>
> >>>>>>> we noticed the issue does not always happen. 34 times out of 50 runs as below.
> >>>>>>> the parent is clean.
> >>>>>>>
> >>>>>>> 1803d0c5ee1a3bbe efa7df3e3bb5da8e6abbe377274
> >>>>>>> ---------------- ---------------------------
> >>>>>>>           fail:runs  %reproduction    fail:runs
> >>>>>>>               |             |             |
> >>>>>>>               :50          68%          34:50    dmesg.Kernel_panic-not_syncing:Fatal_exception
> >>>>>>>               :50          68%          34:50    dmesg.RIP:try_get_folio
> >>>>>>>               :50          68%          34:50    dmesg.invalid_opcode:#[##]
> >>>>>>>               :50          68%          34:50    dmesg.kernel_BUG_at_include/linux/page_ref.h
> >>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> >>>>>>> the same patch/commit), kindly add following tags
> >>>>>>> | Reported-by: kernel test robot <oliver.sang@...el.com>
> >>>>>>> | Closes: https://lore.kernel.org/oe-lkp/202405311534.86cd4043-lkp@intel.com
> >>>>>>>
> >>>>>>>
> >>>>>>> [  275.267158][ T4335] ------------[ cut here ]------------
> >>>>>>> [  275.267949][ T4335] kernel BUG at include/linux/page_ref.h:275!
> >>>>>>> [  275.268526][ T4335] invalid opcode: 0000 [#1] KASAN PTI
> >>>>>>> [  275.269001][ T4335] CPU: 0 PID: 4335 Comm: trinity-c3 Not tainted 6.7.0-rc4-00061-gefa7df3e3bb5 #1
> >>>>>>> [  275.269787][ T4335] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> >>>>>>> [ 275.270679][ T4335] RIP: 0010:try_get_folio (include/linux/page_ref.h:275 (discriminator 3) mm/gup.c:79 (discriminator 3))
> >>>>>>> [ 275.271159][ T4335] Code: c3 cc cc cc cc 44 89 e6 48 89 df e8 e4 54 11 00 eb ae 90 0f 0b 90 31 db eb d5 9c 58 0f 1f 40 00 f6 c4 02 0f 84 46 ff ff ff 90 <0f> 0b 48 c7 c6 a0 54 d2 87 48 89 df e8 a9 e9 ff ff 90 0f 0b be 04
> >>>>>>
> >>>>>> If I read this BUG correctly, it is:
> >>>>>>
> >>>>>> VM_BUG_ON(!in_atomic() && !irqs_disabled());
> >>>>>>
> >>>>>
> >>>>> Yes, that seems to be the one.
> >>>>>
> >>>>>> try_grab_folio() actually assumes it is in an atomic context (irq
> >>>>>> disabled or preempt disabled) for this call path. This is achieved by
> >>>>>> disabling irq in gup fast or calling it in rcu critical section in
> >>>>>> page cache lookup path
> >>>>>
> >>>>> try_grab_folio()->try_get_folio()->folio_ref_try_add_rcu()
> >>>>>
> >>>>> Is called (mm-unstable) from:
> >>>>>
> >>>>> (1) gup_fast function, here IRQs are disable
> >>>>> (2) gup_hugepte(), possibly problematic
> >>>>> (3) memfd_pin_folios(), possibly problematic
> >>>>> (4) __get_user_pages(), likely problematic
> >>>>>
> >>>>> (1) should be fine.
> >>>>>
> >>>>> (2) is possibly problematic on the !fast path. If so, due to commit
> >>>>>        a12083d721d7 ("mm/gup: handle hugepd for follow_page()") ? CCing Peter.
> >>>>>
> >>>>> (3) is possibly wrong. CCing Vivek.
> >>>>>
> >>>>> (4) is what we hit here
> >>>>>
> >>>>>>
> >>>>>> And try_grab_folio() is used when the folio is a large folio. The
> >>>>>
> >>>>>
> >>>>> We come via process_vm_rw()->pin_user_pages_remote()->__get_user_pages()->try_grab_folio()
> >>>>>
> >>>>> That code was added in
> >>>>>
> >>>>> commit 57edfcfd3419b4799353d8cbd6ce49da075cfdbd
> >>>>> Author: Peter Xu <peterx@...hat.com>
> >>>>> Date:   Wed Jun 28 17:53:07 2023 -0400
> >>>>>
> >>>>>        mm/gup: accelerate thp gup even for "pages != NULL"
> >>>>>
> >>>>>        The acceleration of THP was done with ctx.page_mask, however it'll be
> >>>>>        ignored if **pages is non-NULL.
> >>>>>
> >>>>>
> >>>>> Likely the try_grab_folio() in __get_user_pages() is wrong?
> >>>>>
> >>>>> As documented, we already hold a refcount. Likely we should better do a
> >>>>> folio_ref_add() and sanity check the refcount.
> >>>>
> >>>> Yes, a plain folio_ref_add() seems ok for these cases.
> >>>>
> >>>> In addition, the comment of folio_try_get_rcu() says, which is just a
> >>>> wrapper of folio_ref_try_add_rcu():
> >>>>
> >>>> You can also use this function if you're holding a lock that prevents
> >>>> pages being frozen & removed; eg the i_pages lock for the page cache
> >>>> or the mmap_lock or page table lock for page tables.  In this case, it
> >>>> will always succeed, and you could have used a plain folio_get(), but
> >>>> it's sometimes more convenient to have a common function called from
> >>>> both locked and RCU-protected contexts.
> >>>>
> >>>> So IIUC we can use the plain folio_get() at least for
> >>>> process_vm_readv/writev since mmap_lock is held in this path.
> >>>>
> >>>>>
> >>>>>
> >>>>> In essence, I think: try_grab_folio() should only be called from GUP-fast where
> >>>>> IRQs are disabled.
> >>>>
> >>>> Yes, I agree. Just the fast path should need to call try_grab_folio().
> >>>
> >>> try_grab_folio() also handles FOLL_PIN and FOLL_GET, so we may just
> >>> keep calling it and add a flag to try_grab_folio, just like:
> >>>
> >>> if flag is true
> >>>       folio_ref_add()
> >>> else
> >>>       try_get_folio()
> >>
> >>
> >> try_grab_page() is what we use on the GUP-slow path. We'd likely want a
> >> folio variant of that.
> >>
> >> We might want to call that gup_try_grab_folio() and rename the other one
> >> to gup_fast_try_grab_folio().
> >
> > Won't we duplicate the most code with two versions try_grab_folio()?
> >
> > I meant something like:
> >
> > try_grab_folio(struct page *page, int refs, unsigned int flags, bool fast)
> > {
> >      if fast
> >          try_get_folio()
> >      else
> >          folio_ref_add()
> > }
> >
>
> That's insufficient to handle FOLL_PIN. Likely we should do this:
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 231711efa390d..fea93a64bf235 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -203,8 +203,8 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>   }
>
>   /**
> - * try_grab_page() - elevate a page's refcount by a flag-dependent amount
> - * @page:    pointer to page to be grabbed
> + * try_grab_folio() - elevate a folios's refcount by a flag-dependent amount
> + * @folio:   pointer to folio to be grabbed
>    * @flags:   gup flags: these are the FOLL_* flag values.
>    *
>    * This might not do anything at all, depending on the flags argument.
> @@ -216,16 +216,16 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>    * time. Cases: please see the try_grab_folio() documentation, with
>    * "refs=1".
>    *
> + * Must not be called from GUP-fast: the folio must not get freed concurrently.
> + *
>    * Return: 0 for success, or if no action was required (if neither FOLL_PIN
>    * nor FOLL_GET was set, nothing is done). A negative error code for failure:
>    *
>    *   -ENOMEM           FOLL_GET or FOLL_PIN was set, but the page could not
>    *                     be grabbed.
>    */
> -int __must_check try_grab_page(struct page *page, unsigned int flags)
> +int __must_check try_grab_page(struct folio *folio, unsigned int flags)
>   {
> -       struct folio *folio = page_folio(page);
> -
>          if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
>                  return -ENOMEM;
>
> @@ -239,7 +239,7 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
>                   * Don't take a pin on the zero page - it's not going anywhere
>                   * and it is used in a *lot* of places.
>                   */
> -               if (is_zero_page(page))
> +               if (is_zero_folio(folio))
>                          return 0;
>
>                  /*
> @@ -260,6 +260,11 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
>          return 0;
>   }
>
> +int __must_check try_grab_page(struct page *page, unsigned int flags)
> +{
> +       return gup_try_grab_folio(page_folio(page), flags);
> +}
> +
>   /**
>    * unpin_user_page() - release a dma-pinned page
>    * @page:            pointer to page to be released
>
>
> Then, fix the callers and rename the other one to gup_fast_*.

I see your point. Replace try_grab_page() to try_grab_folio() for slow
path, it returns 0 or errno, but it should never fail in slow path
since we already hold at least one reference IIUC. The fast version
should just like old try_grab_folio(), which returns the pointer to
folio or NULL.

>
>
> --
> Cheers,
>
> David / dhildenb
>

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