linux-kernel - Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap

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Date:   Wed, 29 Mar 2023 12:33:05 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v3 1/2] mm: vmalloc: Remove a global vmap_blocks xarray

On 03/28/23 at 02:34pm, Uladzislau Rezki wrote:
......  
> > > @@ -2003,8 +2037,8 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
> > >  	bitmap_set(vb->used_map, 0, (1UL << order));
> > >  	INIT_LIST_HEAD(&vb->free_list);
> > >  
> > > -	vb_idx = addr_to_vb_idx(va->va_start);
> > > -	err = xa_insert(&vmap_blocks, vb_idx, vb, gfp_mask);
> > > +	vbq = addr_to_vbq(va->va_start);
> > > +	err = xa_insert(&vbq->vmap_blocks, va->va_start, vb, gfp_mask);
> > 
> > Using va->va_start as index to access xarray may cost extra memory.
> > Imagine we got a virtual address at VMALLOC_START, its region is
> > [VMALLOC_START, VMALLOC_START+4095]. In the xarray, its sequence order
> > is 0. While with va->va_start, it's 0xffffc90000000000UL on x86_64 with
> > level4 paging mode. That means for the first page size vmalloc area,
> > storing it into xarray need about 10 levels of xa_node, just for the one
> > page size. With the old addr_to_vb_idx(), its index is 0. Only one level
> > height is needed. One xa_node is about 72bytes, it could take more time
> > and memory to access va->va_start. Not sure if my understanding is correct.
> > 
> > static unsigned long addr_to_vb_idx(unsigned long addr)
> > {
> >         addr -= VMALLOC_START & ~(VMAP_BLOCK_SIZE-1);
> >         addr /= VMAP_BLOCK_SIZE;
> >         return addr;
> > }
> > 
> If the size of array depends on index "length", then, indeed it will require
> more memory. From the other hand we can keep the old addr_to_vb_idx() function 
> in order to "cut" a va->va_start index.

Yeah, the extra 10 levels of xa_node is unnecessary if we keep the old
addr_to_vb_idx(). And the prolonged path will cost more time to reach the 
wanted leaf node. E.g on x86_64 with 4 level paging mode, vmalloc area
is 32TB. With the old calculation, its index range is [0, 8M], 4 level
heights of xa_node at most is enough to cover.