linux-kernel - Re: [PATCH] zsmalloc: use actual object size to detect spans

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Message-ID: <vkhwvmazwys7dz7m67fil7xlhevdsc4xbvq3h6nknb7rqojjis@vx2c57ylvl44>
Date: Wed, 7 Jan 2026 10:37:24 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Nhat Pham <nphamcs@...il.com>, Minchan Kim <minchan@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Brian Geffon <bgeffon@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] zsmalloc: use actual object size to detect spans

On (26/01/07 09:59), Sergey Senozhatsky wrote:
> On (26/01/07 00:23), Yosry Ahmed wrote:
> > Instead of modifying mem_len, can we modify 'off' like zs_obj_write()
> > and zs_obj_read_end()? I think this can actually be done as a prequel to
> > this patch. Arguably, it makes more sense as we avoid unnecessarily
> > copying the handle (completely untested):
> > 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 5bf832f9c05c..48c288da43b8 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1087,6 +1087,9 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
> >         class = zspage_class(pool, zspage);
> >         off = offset_in_page(class->size * obj_idx);
> > 
> > +       if (!ZsHugePage(zspage))
> > +               off += ZS_HANDLE_SIZE;
> > +
> >         if (off + class->size <= PAGE_SIZE) {
> >                 /* this object is contained entirely within a page */
> >                 addr = kmap_local_zpdesc(zpdesc);
> > @@ -1107,9 +1110,6 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
> >                                  0, sizes[1]);
> >         }
> > 
> > -       if (!ZsHugePage(zspage))
> > -               addr += ZS_HANDLE_SIZE;
> > -
> >         return addr;
> >  }
> >  EXPORT_SYMBOL_GPL(zs_obj_read_begin);
> > @@ -1129,9 +1129,10 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
> >         class = zspage_class(pool, zspage);
> >         off = offset_in_page(class->size * obj_idx);
> > 
> > +       if (!ZsHugePage(zspage))
> > +               off += ZS_HANDLE_SIZE;
> > +
> >         if (off + class->size <= PAGE_SIZE) {
> > -               if (!ZsHugePage(zspage))
> > -                       off += ZS_HANDLE_SIZE;
> >                 handle_mem -= off;
> >                 kunmap_local(handle_mem);
> >         }
> > 
> > ---
> > Does this work?
> 
> Sounds interesting.  Let me try it out.

I recall us having exactly this idea when we first introduced
zs_obj_{read,write}_end() functions, and I do recall that it
did not work.  Somehow this panics in __memcpy+0xc/0x44.  Let
me dig into it again.