Hi Michael:
	Thanks for your review.

From: Michael Kelley <redacted>
Sent: Sunday, January 19, 2020 3:29 AM
To: lantianyu1986@gmail.com; KY Srinivasan <kys@microsoft.com>; Haiyang
Zhang [off-list ref]; Stephen Hemminger
[off-list ref]; sashal@kernel.org
Cc: Tianyu Lan <redacted>; linux-hyperv@vger.kernel.org;
linux-kernel@vger.kernel.org; vkuznets [off-list ref];
stable@vger.kernel.org
Subject: RE: [PATCH V2] x86/Hyper-V: Balloon up according to request page
number

From: Tianyu Lan <redacted> Sent: Thursday, January 16,
2020 6:16 AM

quoted

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.

quoted

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@vger.kernel.org
Signed-off-by: Tianyu Lan <redacted>
---
Change since v2:
    - Change logic of switching alloc_unit from 2MB to 4KB
    in the balloon_up() to avoid redundant iteration when
    handle non-aligned page request.
    - Remove 2MB alignment operation and comment in balloon_up()
---
 drivers/hv/hv_balloon.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c index

7f3e7ab22d5d..536807efbc35 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;

The above test is no longer necessary.  The num_pages < alloc_unit case is
handled implicitly by your new 'for' loop condition.

Yes, will update in the next version.

quoted

-	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.

@@ -1749,14 +1746,13 @@ static void balloon_up(union dm_msg_info

*msg_info)

quoted

 	avail_pages = si_mem_available();
 	floor = compute_balloon_floor();

-	/* Refuse to balloon below the floor, keep the 2M granularity. */
+	/* Refuse to balloon below the floor. */
 	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;
 	}

 	while (!done) {

@@ -1770,7 +1766,7 @@ static void balloon_up(union dm_msg_info

*msg_info)

quoted

 		num_ballooned = alloc_balloon_pages(&dm_device,

num_pages,

quoted

 						    bl_resp, alloc_unit);

-		if (alloc_unit != 1 && num_ballooned == 0) {
+		if (alloc_unit != 1 && num_ballooned != num_pages) {

Maybe I'm missing something, but I don't think Vitaly's optimization works.  If
alloc_unit specifies 2 Mbytes, and num_pages specifies 3 Mbytes, then
num_ballooned will come back as 2 Mbytes, which is correct.  But if we revert
alloc_unit to 1 page and "continue" in that case, we will lose the 2 Mbytes of
memory (it's not freed), and the next time through the loop will try to allocate
only 1 Mbyte (because num_pages will be decremented by num_ballooned).  I
think the original code does the right thing.

Michael

Sorry. I should remove the "continue" here and then it will work.  Will fix this in the
next version.

quoted

 			alloc_unit = 1;
 			continue;
 		}
--
2.14.5