linux-kernel - Re: [RFC PATCH v2] mm/vmalloc: fix incorrect __vmap_pages_range_noflush() if vm_area_alloc_pages() from high order fallback to order0

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Message-ID: <CAGsJ_4xF2J=xxG1rVvFARJtVrvfrGAr7ZLtMCgp8obL6Q0TcMw@mail.gmail.com>
Date: Thu, 25 Jul 2024 21:34:52 +1200
From: Barry Song <21cnbao@...il.com>
To: Hailong Liu <hailong.liu@...o.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Uladzislau Rezki <urezki@...il.com>, 
	Christoph Hellwig <hch@...radead.org>, Lorenzo Stoakes <lstoakes@...il.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Michal Hocko <mhocko@...e.com>, Baoquan He <bhe@...hat.com>, Matthew Wilcox <willy@...radead.org>, 
	"Tangquan . Zheng" <zhengtangquan@...o.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2] mm/vmalloc: fix incorrect __vmap_pages_range_noflush()
 if vm_area_alloc_pages() from high order fallback to order0

On Thu, Jul 25, 2024 at 9:17 PM Hailong Liu <hailong.liu@...o.com> wrote:
>
> On Thu, 25. Jul 18:21, Barry Song wrote:
> > On Thu, Jul 25, 2024 at 3:53 PM <hailong.liu@...o.com> wrote:
> [snip]
> >
> > This is still incorrect because it undoes Michal's work. We also need to break
> > the loop if (!nofail), which you're currently omitting.
>
> IIUC, the origin issue is to fix kvcalloc with __GFP_NOFAIL return NULL.
> https://lore.kernel.org/all/ZAXynvdNqcI0f6Us@dhcp22.suse.cz/T/#u
> if we disable huge flag in kmalloc_node, the issue will be fixed.

No, this just bypasses kvmalloc and doesn't solve the underlying issue. Problems
can still be triggered by vmalloc_huge() even after the bypass. Once we
reorganize vmap_huge to support the combination of PMD and PTE
mapping, we should re-enable HUGE_VMAP for kvmalloc.

I would consider dropping VM_ALLOW_HUGE_VMAP() for kvmalloc as
an short-term "optimization" to save memory rather than a long-term fix. This
'optimization' is only valid until we reorganize HUGE_VMAP in a way
similar to THP. I mean, for a 2.1MB kvmalloc, we can map 2MB as PMD
and 0.1 as PTE.

> >
> > To avoid reverting Michal's work, the simplest "fix" would be,
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index caf032f0bd69..0011ca30df1c 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3775,7 +3775,7 @@ void *__vmalloc_node_range_noprof(unsigned long
> > size, unsigned long align,
> >                 return NULL;
> >         }
> >
> > -       if (vmap_allow_huge && (vm_flags & VM_ALLOW_HUGE_VMAP)) {
> > +       if (vmap_allow_huge && (vm_flags & VM_ALLOW_HUGE_VMAP) &
> > !(gfp_mask & __GFP_NOFAIL)) {
> >                 unsigned long size_per_node;
> >
> >                 /*
> > >
> > > [1] https://lore.kernel.org/lkml/20240724182827.nlgdckimtg2gwns5@oppo.com/
> > > 2.34.1
> >
> > Thanks
> > Barry
>
> --
> help you, help me,
> Hailong.