Hi Vitaly:
Thanks for your review. I fix comments in V2.
> From: linux-hyperv-owner@xxxxxxxxxxxxxxx <linux-hyperv-
> owner@xxxxxxxxxxxxxxx> On Behalf Of Vitaly Kuznetsov
> Sent: Tuesday, January 14, 2020 5:31 PM
> To: lantianyu1986@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; linux-
> mm@xxxxxxxxx; stable@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx;
> mingo@xxxxxxxxxx; bp@xxxxxxxxx; hpa@xxxxxxxxx; x86@xxxxxxxxxx;
> dave.hansen@xxxxxxxxxxxxxxx; luto@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; KY
> Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; sashal@xxxxxxxxxx;
> akpm@xxxxxxxxxxxxxxxxxxxx; Michael Kelley <mikelley@xxxxxxxxxxxxx>;
> Dexuan Cui <decui@xxxxxxxxxxxxx>
> Subject: [EXTERNAL] Re: [PATCH] x86/Hyper-V: Balloon up according to request
> page number
>
> lantianyu1986@xxxxxxxxx writes:
>
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > Current code has assumption that balloon request memory size aligns
> > with 2MB. But actually Hyper-V doesn't guarantee such alignment. When
> > balloon driver receives non-aligned balloon request, it produces
> > warning and balloon up more memory than requested in order to keep 2MB
> alignment.
> > Remove the warning and balloon up memory according to actual requested
> > memory size.
> >
> > Fixes: f6712238471a ("hv: hv_balloon: avoid memory leak on alloc_error
> > of 2MB memory block")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > ---
> > drivers/hv/hv_balloon.c | 7 ++-----
> > 1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c index
> > 7f3e7ab22d5d..38ad0e44e927 100644
> > --- a/drivers/hv/hv_balloon.c
> > +++ b/drivers/hv/hv_balloon.c
> > @@ -1684,7 +1684,7 @@ static unsigned int alloc_balloon_pages(struct
> hv_dynmem_device *dm,
> > if (num_pages < alloc_unit)
> > return 0;
> >
> > - for (i = 0; (i * alloc_unit) < num_pages; i++) {
> > + for (i = 0; i < num_pages / alloc_unit; i++) {
> > if (bl_resp->hdr.size + sizeof(union dm_mem_page_range) >
> > HV_HYP_PAGE_SIZE)
> > return i * alloc_unit;
> > @@ -1722,7 +1722,7 @@ static unsigned int alloc_balloon_pages(struct
> > hv_dynmem_device *dm,
> >
> > }
> >
> > - return num_pages;
> > + return i * alloc_unit;
> > }
> >
> > static void balloon_up(union dm_msg_info *msg_info) @@ -1737,9
> > +1737,6 @@ static void balloon_up(union dm_msg_info *msg_info)
> > long avail_pages;
> > unsigned long floor;
> >
> > - /* The host balloons pages in 2M granularity. */
> > - WARN_ON_ONCE(num_pages % PAGES_IN_2M != 0);
> > -
> > /*
> > * We will attempt 2M allocations. However, if we fail to
> > * allocate 2M chunks, we will go back to PAGE_SIZE allocations.
>
> This looks correct but I've noticed we also have
>
> /* Refuse to balloon below the floor, keep the 2M granularity. */
> if (avail_pages < num_pages || avail_pages - num_pages < floor) {
> pr_warn("Balloon request will be partially fulfilled. %s\n",
> avail_pages < num_pages ? "Not enough memory." :
> "Balloon floor reached.");
>
> num_pages = avail_pages > floor ? (avail_pages - floor) : 0;
> num_pages -= num_pages % PAGES_IN_2M;
> }
>
> in balloon_up(). If 2M granularity is not guaranteed in the first place we can't
> keep it.
>
> Also, when alloc_balloon_pages() is called with 2M alloc_unit and the region is
> not 2M aligned, it will return someething < num_pages, the next condition,
> however, only checks for 0:
>
> if (alloc_unit != 1 && num_ballooned == 0) {
> alloc_unit = 1;
> continue;
> }
>
> we will proceed to sending a response to server and try doing next iteration by
> calling alloc_balloon_pages() with 2M alloc_unit again, this will finally return 0
> and we will switch to 4k. I think we can optimize this to:
>
> if (alloc_unit != 1 && num_ballooned != num_pages) {
> alloc_unit = 1;
> continue;
> }
>
> --
> Vitaly
