RE: [PATCH] fbdev: hyperv_fb: Allow graceful removal of framebuffer

[Michael Kelley]

From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Sunday, February 23, 2025 6:10 AM
> 
> On Sat, Feb 22, 2025 at 08:16:53PM +0000, Michael Kelley wrote:
> > From: Saurabh Singh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Saturday, February 22, 2025 9:27 AM
> > >

[anip]

> > >
> > > I had considered moving the entire `hvfb_putmem()` function to `destroy`,
> > > but I was hesitant for two reasons:
> > >
> > >   1. I wasn’t aware of any scenario where this would be useful. However,
> > >      your explanation has convinced me that it is necessary.
> > >   2. `hvfb_release_phymem()` relies on the `hdev` pointer, which requires
> > >      multiple `container_of` operations to derive it from the `info` pointer.
> > >      I was unsure if the complexity was justified, but it seems worthwhile now.
> > >
> > > I will move `hvfb_putmem()` to the `destroy` function in V2, and I hope this
> > > will address all the cases you mentioned.
> > >
> >
> > Yes, that's what I expect needs to happen, though I haven't looked at the
> > details of making sure all the needed data structures are still around. Like
> > you, I just had this sense that hvfb_putmem() might need to be moved as
> > well, so I tried to produce a failure scenario to prove it, which turned out
> > to be easy.
> >
> > Michael
> 
> I will add this in V2 as well. But I have found an another issue which is
> not very frequent.
> 
> 
> [  176.562153] ------------[ cut here ]------------
> [  176.562159] fb0: fb_WARN_ON_ONCE(pageref->page != page)
> [  176.562176] WARNING: CPU: 50 PID: 1522 at drivers/video/fbdev/core/fb_defio.c:67
> fb_deferred_io_mkwrite+0x215/0x280
> 
> <snip>
> 
> [  176.562258] Call Trace:
> [  176.562260]  <TASK>
> [  176.562263]  ? show_regs+0x6c/0x80
> [  176.562269]  ? __warn+0x8d/0x150
> [  176.562273]  ? fb_deferred_io_mkwrite+0x215/0x280
> [  176.562275]  ? report_bug+0x182/0x1b0
> [  176.562280]  ? handle_bug+0x133/0x1a0
> [  176.562283]  ? exc_invalid_op+0x18/0x80
> [  176.562284]  ? asm_exc_invalid_op+0x1b/0x20
> [  176.562289]  ? fb_deferred_io_mkwrite+0x215/0x280
> [  176.562291]  ? fb_deferred_io_mkwrite+0x215/0x280
> [  176.562293]  do_page_mkwrite+0x4d/0xb0
> [  176.562296]  do_wp_page+0xe8/0xd50
> [  176.562300]  ? ___pte_offset_map+0x1c/0x1b0
> [  176.562304]  __handle_mm_fault+0xbe1/0x10e0
> [  176.562307]  handle_mm_fault+0x17f/0x2e0
> [  176.562309]  do_user_addr_fault+0x2d1/0x8d0
> [  176.562314]  exc_page_fault+0x85/0x1e0
> [  176.562318]  asm_exc_page_fault+0x27/0x30
> 
> Looks this is because driver is unbind still Xorg is trying to write
> to memory which is causing some page faults. I have confirmed PID 1522
> is of Xorg. I think this is because we need to cancel the framebuffer
> deferred work after flushing it.

Does this new issue occur even after moving hvfb_putmem()
into the destroy() function?  I'm hoping it doesn't. I've
looked at the fb_deferred_io code, and can't quite figure out
how that deferred I/O work is supposed to get cancelled. Or
maybe it's just not supposed to get started again after the flush.

If the new issue still happens, that seems like more of a flaw
in the fb deferred I/O mechanism not shutting itself down
properly.

Michael

> 
> After adding the below in hvfb_remove I don't see this issue anymore.
> Although as the issue is not very frequent I am not 100% sure.
> 
> 	cancel_delayed_work_sync(&info->deferred_work);
> 
> If you think this is reasonable I can add this as well in V2.
>