Thank you Andrew for the reply.
On 5/12/2020 1:41 AM, Andrew Morton wrote: > On Mon, 11 May 2020 19:10:08 +0530 Charan Teja Reddy > <chara...@codeaurora.org> wrote: > >> Updating the zone watermarks by any means, like extra_free_kbytes, >> min_free_kbytes, water_mark_scale_factor e.t.c, when watermark_boost is >> set will result into the higher low and high watermarks than the user >> asks. This can be avoided by resetting the zone->watermark_boost to zero >> early. > > Does this solve some problem which has been observed in testing? Sorry, what are those issues observed in testing? It would be helpful If you post them here. > >> ... >> >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -7746,9 +7746,9 @@ static void __setup_per_zone_wmarks(void) >> mult_frac(zone_managed_pages(zone), >> watermark_scale_factor, 10000)); >> >> + zone->watermark_boost = 0; >> zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp; >> zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2; >> - zone->watermark_boost = 0; >> >> spin_unlock_irqrestore(&zone->lock, flags); >> } > > This could only be a problem if code is accessing these things without > holding zone->lock. Is that ever the case? > This is a problem even when accessing these things with zone->lock held because we are directly using the macro min_wmark_pages(zone) which leads to the issue. Pasting macro here for reference. #define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost) Steps that lead to the issue is like below: 1) On the extfrag event, we try to boost the watermark by storing the value in ->watermark_boost. 2) User changes the value of extra|min_free_kbytes or watermark_scale_factor. In __setup_perzone_wmarks, we directly store the user asked watermarks in the zones structure. In this step, the value is always offsets by ->watermark_boost as we use the min_wmark_pages() macro. 3) Later, when kswapd woke up, it resets the zone's watermark_boost to zero. Step 2 from the above is what resulting into the issue. -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
