Re: [Xen-devel] workaround for dom0 crash due to QEMU using O_DIRECT

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Use the new directiosafe option, or simply:

diff --git a/hw/xen_disk.c b/hw/xen_disk.c
index 4108ce8..70c9ea0 100644
--- a/hw/xen_disk.c
+++ b/hw/xen_disk.c
@@ -790,12 +790,7 @@ static int blk_connect(struct XenDevice *xendev)
     struct XenBlkDev *blkdev = container_of(xendev, struct XenBlkDev, xendev);
     int pers, index, qflags;
 
-    /* read-only ? */
-    if (blkdev->directiosafe) {
-        qflags = BDRV_O_NOCACHE | BDRV_O_NATIVE_AIO;
-    } else {
-        qflags = BDRV_O_CACHE_WB;
-    }
+    qflags = BDRV_O_NOCACHE | BDRV_O_NATIVE_AIO;
     if (strcmp(blkdev->mode, "w") == 0) {
         qflags |= BDRV_O_RDWR;
     }


On Thu, 4 Jul 2013, Alex Bligh wrote:
> Stefano,
> 
> We'll test this (or more precisely Diana will when she has a minute). Remind
> me how you'd like O_DIRECT re-enabled. Back out the patch?
> 
> Alex
> 
> --On 4 July 2013 19:19:40 +0100 Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 
> > Hi Alex,
> > speaking with Ian about the dom0 kernel crash caused by using O_DIRECT
> > in QEMU, we came up with a simple workaround that should turn the crash
> > into a data corruption problem (same as native).
> > 
> > The idea is that when we balloon out pages, we replace the original page
> > with a mapping of a scrub page, so that if the network stack wants to
> > access an old grant that doesn't exist anymore, it should find a valid
> > page mapped there (the scrub page).
> > 
> > Could you please try the appended patch for Linux with QEMU that uses
> > O_DIRECT to open a file on NFS?
> > 
> > Thanks!
> > 
> > - Stefano
> > 
> > ---
> > 
> > 
> > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > index 930fb68..0663fda 100644
> > --- a/drivers/xen/balloon.c
> > +++ b/drivers/xen/balloon.c
> > @@ -88,6 +88,7 @@ EXPORT_SYMBOL_GPL(balloon_stats);
> > 
> >  /* We increase/decrease in batches which fit in a page */
> >  static xen_pfn_t frame_list[PAGE_SIZE / sizeof(unsigned long)];
> > +static struct page* trade_page;
> > 
> >  #ifdef CONFIG_HIGHMEM
> >  #define inc_totalhigh_pages() (totalhigh_pages++)
> > @@ -423,7 +424,7 @@ static enum bp_state decrease_reservation(unsigned
> > long nr_pages, gfp_t gfp)           if (xen_pv_domain() &&
> > !PageHighMem(page)) {
> >                     ret = HYPERVISOR_update_va_mapping(
> >                             (unsigned long)__va(pfn << PAGE_SHIFT),
> > -                           __pte_ma(0), 0);
> > +                           pfn_pte(page_to_pfn(trade_page), PAGE_KERNEL),
> > 0);
> >                     BUG_ON(ret);
> >             }
> >  #endif
> > @@ -436,7 +437,7 @@ static enum bp_state decrease_reservation(unsigned
> > long nr_pages, gfp_t gfp)   /* No more mappings: invalidate P2M and add
> > to balloon. */
> >     for (i = 0; i < nr_pages; i++) {
> >             pfn = mfn_to_pfn(frame_list[i]);
> > -           __set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
> > +           __set_phys_to_machine(pfn,
> > pfn_to_mfn(page_to_pfn(trade_page)));
> >             balloon_append(pfn_to_page(pfn));
> >     }
> > 
> > @@ -591,6 +592,10 @@ static int __init balloon_init(void)
> >     if (!xen_domain())
> >             return -ENODEV;
> > 
> > +   trade_page = alloc_page(GFP_KERNEL);
> > +   if (trade_page == NULL)
> > +           return -ENOMEM;
> > +
> >     pr_info("xen/balloon: Initialising balloon driver.\n");
> > 
> >     balloon_stats.current_pages = xen_pv_domain()
> > 
> > 
> 
> 
> 
> -- 
> Alex Bligh
> 

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