linux-kernel - RE: [PATCH] video: hyperv_fb: Directly use the MMIO VRAM

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Message-ID: <MW2PR2101MB1801841901E659E60502EB86BFFB0@MW2PR2101MB1801.namprd21.prod.outlook.com>
Date:   Tue, 24 Nov 2020 08:33:32 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Boqun Feng <boqun.feng@...il.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "b.zolnierkie@...sung.com" <b.zolnierkie@...sung.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Hu <weh@...rosoft.com>
Subject: RE: [PATCH] video: hyperv_fb: Directly use the MMIO VRAM

Hi Wei Liu,
Please do not pick up this patch, because actually MMIO VRAM can not work
with fb_deferred_io.

Previously I didn't test Xorg -- sorry. As soon as I tested it, I got the below
warning and the Xorg program ternimated immediately:

[   28.148432] WARNING: CPU: 19 PID: 1410 at mm/vmalloc.c:383 vmalloc_to_page+0x14b/0x150
...
[   28.192959] CPU: 19 PID: 1410 Comm: Xorg Tainted: G            E     5.10.0-rc1+ #4
...
[   28.208720] RIP: 0010:vmalloc_to_page+0x14b/0x150
...
[   28.299231] Call Trace:
[   28.301428]  fb_deferred_io_fault+0x3a/0xa0
[   28.305276]  __do_fault+0x36/0x120
[   28.308276]  handle_mm_fault+0x1144/0x1950
[   28.311963]  exc_page_fault+0x290/0x510
[   28.315551]  ? asm_exc_page_fault+0x8/0x30
[   28.319186]  asm_exc_page_fault+0x1e/0x30
[   28.322969] RIP: 0033:0x7fbeda3ec2f5

The issue is that fb_deferred_io_page() requires that the PFN be backed by a
struct page, but it looks the MMIO address does not have the struct page backed.

So I have to drop this patch. 
Thanks Wei Hu and Michael for pointing this out!

FYI: drivers/video/fbdev/core/fb_defio.c:
static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
{
        void *screen_base = (void __force *) info->screen_base;
        struct page *page;

        if (is_vmalloc_addr(screen_base + offs))
                page = vmalloc_to_page(screen_base + offs);
        else
                page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT);

        return page;
}

/* this is to find and return the vmalloc-ed fb pages */
static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
{
        unsigned long offset;
        struct page *page;
        struct fb_info *info = vmf->vma->vm_private_data;

        offset = vmf->pgoff << PAGE_SHIFT;
        if (offset >= info->fix.smem_len)
                return VM_FAULT_SIGBUS;

        page = fb_deferred_io_page(info, offset);
        if (!page)
                return VM_FAULT_SIGBUS;

Thanks,
-- Dexuan