linux-kernel - Re: [PATCH v2 2/2] mm/vmalloc: Print a warning message first on failure

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Date:   Mon, 10 May 2021 12:33:42 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Mel Gorman <mgorman@...e.de>, Hillf Danton <hdanton@...a.com>,
        Michal Hocko <mhocko@...e.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 2/2] mm/vmalloc: Print a warning message first on
 failure

On Sun, May 09, 2021 at 11:26:41PM +0200, Uladzislau Rezki wrote:
> On Sun, May 09, 2021 at 09:18:46PM +0100, Matthew Wilcox wrote:
> > On Sun, May 09, 2021 at 10:05:19PM +0200, Uladzislau Rezki wrote:
> > > On Sun, May 09, 2021 at 08:47:12PM +0100, Matthew Wilcox wrote:
> > > > > +++ b/mm/vmalloc.c
> > > > > @@ -2781,11 +2781,11 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
> > > > >  	}
> > > > >  
> > > > >  	if (!area->pages) {
> > > > > -		free_vm_area(area);
> > > > >  		warn_alloc(gfp_mask, NULL,
> > > > >  			   "vmalloc size %lu allocation failure: "
> > > > >  			   "page array size %lu allocation failed",
> > > > >  			   nr_small_pages * PAGE_SIZE, array_size);
> > > > > +		free_vm_area(area);
> > > > >  		return NULL;
> > > > >  	}
> > > > 
> > > > I think this is a bad idea.  We're clearly low on memory (a memory
> > > > allocation just failed).  We should free the memory we have allocated
> > > > to improve the chances of the warning message making it out.
> > > Not sure if i fully follow you here. We do free the memory. The intention
> > > was to print a warning message first because, if, potentially, the
> > > free_vm_area(area) also does some prints it would be a bit messy from the
> > > point what has been broken first.
> > > 
> > > So, could you please clarify what was your concern?
> > 
> > We may need to allocate memory in order to emit the error message.
> >
> > Your commit message didn't mention the potential confusion, and I think
> > that is worth adding for a v4.
> I agree that the commit message should be updated in regard of potential
> confusion, because it was the main intention of this patch.
> 
> I will upload a v4 tomorrow.
> 
Please find the v4 version of the patch that is in question:

>From 7e27e4ac8f299ae244e9e0e90e0292ae2c08d37d Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Date: Sat, 8 May 2021 23:41:21 +0200
Subject: [PATCH v4 1/1] mm/vmalloc: Print a warning message first on failure

When a memory allocation for array of pages are not succeed
emit a warning message as a first step and then perform the
further cleanup.

The reason it should be done in a right order is the clean
up function which is free_vm_area() can potentially also
follow its error paths what can lead to confusion what was
broken first.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index dbc6744400d5..1f664a17d9ea 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2781,11 +2781,11 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
 	}
 
 	if (!area->pages) {
-		free_vm_area(area);
 		warn_alloc(gfp_mask, NULL,
 			   "vmalloc size %lu allocation failure: "
 			   "page array size %lu allocation failed",
 			   nr_small_pages * PAGE_SIZE, array_size);
+		free_vm_area(area);
 		return NULL;
 	}
 
-- 
2.20.1

--
Vlad Rezki