linux-kernel - Re: [PATCH] mm/vmalloc: Remove WARN_ON_ONCE related to adjust_va_to_fit

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Date:   Tue, 26 Sep 2023 08:05:54 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Jaeseon Sim <jason.sim@...sung.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        "bhe@...hat.com" <bhe@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "lstoakes@...il.com" <lstoakes@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>
Subject: Re: [PATCH] mm/vmalloc: Remove WARN_ON_ONCE related to
 adjust_va_to_fit_type

> > We do not have above code anymore:
> Sorry, I tried to say it in a simplified way and it caused a misunderstanding.
> 
> <snip>
> static __always_inline int
> adjust_va_to_fit_type(struct rb_root *root, struct list_head *head,
> 		      struct vmap_area *va, unsigned long nva_start_addr,
> 		      unsigned long size)
> 
> 	} else if (type == NE_FIT_TYPE) {
> 		/*
> 		 * Split no edge of fit VA.
> 		 *
> 		 *     |       |
> 		 *   L V  NVA  V R
> 		 * |---|-------|---|
> 		 */
> 		lva = __this_cpu_xchg(ne_fit_preload_node, NULL);
> 		if (unlikely(!lva)) {
> 			/*
> 			 * For percpu allocator we do not do any pre-allocation
> 			 * and leave it as it is. The reason is it most likely
> 			 * never ends up with NE_FIT_TYPE splitting. In case of
> 			 * percpu allocations offsets and sizes are aligned to
> 			 * fixed align request, i.e. RE_FIT_TYPE and FL_FIT_TYPE
> 			 * are its main fitting cases.
> 			 *
> 			 * There are a few exceptions though, as an example it is
> 			 * a first allocation (early boot up) when we have "one"
> 			 * big free space that has to be split.
> 			 *
> 			 * Also we can hit this path in case of regular "vmap"
> 			 * allocations, if "this" current CPU was not preloaded.
> 			 * See the comment in alloc_vmap_area() why. If so, then
> 			 * GFP_NOWAIT is used instead to get an extra object for
> 			 * split purpose. That is rare and most time does not
> 			 * occur.
> 			 *
> 			 * What happens if an allocation gets failed. Basically,
> 			 * an "overflow" path is triggered to purge lazily freed
> 			 * areas to free some memory, then, the "retry" path is
> 			 * triggered to repeat one more time. See more details
> 			 * in alloc_vmap_area() function.
> 			 */
> 			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
> 			if (!lva)
> 				return -1;
> 		}
> <snip>
> 
> Above allocation fail will meet WARN_ON_ONCE in the current kernel now.
> Should It be handled by alloc_vmap_area()?, as you described in a comment.
> 
WARN_ONCE_ONCE() is a warning and not a panic, though your kernel config
considers it as a panic. Right, we go on retry path and we can remove
the warning only for GFP_NOWAIT-alloc-error. From the other hand we
should still have possibility to trigger a warning if an allocation
is not successful: no vmap space or low memory condition, thus no
physical memory.

> 
> > 
> > <snip>
> > commit 82dd23e84be3ead53b6d584d836f51852d1096e6
> > Author: Uladzislau Rezki (Sony) <urezki@...il.com>
> > Date:   Thu Jul 11 20:58:57 2019 -0700
> > 
> >     mm/vmalloc.c: preload a CPU with one object for split purpose
> > 
> > <snip>
> > 
> > Which kernel are you testing?
> 
> I'm currently testing v6.1. 
> The panic occurred during power on/off test.
>
Could you please describe in more detail your test sequence, setup and HW?

--
Uladzislau Rezki