Date: Fri, 31 May 2024 11:30:45 -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 11:24 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()
}

We can keep the duplicated code minimum in this way.

>
> Or something like that :)
>
> --
> Cheers,
>
> David / dhildenb
>

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