linux-kernel - Re: [PATCH] mm/vmalloc: do not output a spurious warning when huge vmalloc() fails

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Message-ID: <ZH7xCy9QBmMLg9SQ@pc638.lan>
Date:   Tue, 6 Jun 2023 10:40:43 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Linux btrfs <linux-btrfs@...r.kernel.org>,
        Linux Regressions <regressions@...ts.linux.dev>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, a1bert@...as.cz,
        Forza <forza@...nline.net>
Subject: Re: [PATCH] mm/vmalloc: do not output a spurious warning when huge
 vmalloc() fails

On Tue, Jun 06, 2023 at 09:24:33AM +0100, Lorenzo Stoakes wrote:
> On Tue, Jun 06, 2023 at 10:17:02AM +0200, Uladzislau Rezki wrote:
> > On Tue, Jun 06, 2023 at 09:13:24AM +0200, Vlastimil Babka wrote:
> > >
> > > On 6/5/23 22:11, Lorenzo Stoakes wrote:
> > > > In __vmalloc_area_node() we always warn_alloc() when an allocation
> > > > performed by vm_area_alloc_pages() fails unless it was due to a pending
> > > > fatal signal.
> > > >
> > > > However, huge page allocations instigated either by vmalloc_huge() or
> > > > __vmalloc_node_range() (or a caller that invokes this like kvmalloc() or
> > > > kvmalloc_node()) always falls back to order-0 allocations if the huge page
> > > > allocation fails.
> > > >
> > > > This renders the warning useless and noisy, especially as all callers
> > > > appear to be aware that this may fallback. This has already resulted in at
> > > > least one bug report from a user who was confused by this (see link).
> > > >
> > > > Therefore, simply update the code to only output this warning for order-0
> > > > pages when no fatal signal is pending.
> > > >
> > > > Link: https://bugzilla.suse.com/show_bug.cgi?id=1211410
> > > > Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> > >
> > > I think there are more reports of same thing from the btrfs context, that
> > > appear to be a 6.3 regression
> > >
> > > https://bugzilla.kernel.org/show_bug.cgi?id=217466
> > > Link: https://lore.kernel.org/all/efa04d56-cd7f-6620-bca7-1df89f49bf4b@gmail.com/
> > >
> > I had a look at that report. The btrfs complains due to the
> > fact that a high-order page(1 << 9) can not be obtained. In the
> > vmalloc code we do not fall to 0-order allocator if there is
> > a request of getting a high-order.
> 
> This isn't true, we _do_ fallback to order-0 (this is the basis of my patch), in
> __vmalloc_node_range():-
> 
> 	/* Allocate physical pages and map them into vmalloc space. */
> 	ret = __vmalloc_area_node(area, gfp_mask, prot, shift, node);
> 	if (!ret)
> 		goto fail;
> 
> ...
> 
> fail:
> 	if (shift > PAGE_SHIFT) {
> 		shift = PAGE_SHIFT;
> 		align = real_align;
> 		size = real_size;
> 		goto again;
> 	}
> 
> With the order being derived from shift, and __vmalloc_area_node() only being
> called from __vmalloc_node_range().
> 
Correct. It is done on an upper-layer whereas i checked the vm_area_alloc_pages() function.
But as you mentioned, the refactoring has to be done as it looks a bit messy. 

--
Uladzislau Rezki