On Wed, Nov 14, 2012 at 2:48 PM, Geoffrey Garen <gga...@apple.com> wrote:
> Hi Eric. > > Here are some problems in RenderArena that I know of: > > - Grows memory without bound > - Duplicates the functionality of FastMalloc > - Makes allocation error-prone (allocation in the wrong arena is sometimes > a leak, sometimes a use-after-free, and sometimes a heizenbug of the two) > - Makes allocation verbose (you have to thread an arena pointer everywhere) > - Makes object lifetime complicated (all objects are implicitly tied to a > single owner they may outlive) > - Uses C-style macros and manual initialization and destruction, instead > of modern WebKit C++ style > > You didn't mention any of these problems in your email, so I'll assume you > weren't aware of them. > > Considering these problems now, please don't use RenderArena in more > places. > > > Slab-allocators (i.e. RenderArena) hand out memory all from a single > > region, guaranteeing (among other things) that free'd objects can only > > be ever overwritten by other objects from the same pool. This makes > > it much harder, for example to find a Use-After-Free of a RenderBlock > > and then fill that RenderBlock's memory (and vtable pointer) with > > arbitrary memory (like the contents of a javascript array). > > http://en.wikipedia.org/wiki/Slab_allocation > > This is magical thinking. RenderArena is no different from FastMalloc. > > (1) RenderArena recycles by object size, just like FastMalloc. > > (2) FastMalloc is a slab allocator, just like RenderArena. > > (3) RenderArena grows by calling FastMalloc. > > Isolating object types from each other -- and specifically isolating > objects of arbitrary size and contents like arrays -- is an interesting > idea. RenderArena is neither necessary nor sufficient for implementing this > feature. > > The only reason RenderArena seems isolated from other object types is > social, not technical: we actively discourage using RenderArena, so few > object types currently use it. > > > Since RenderArena is generic, the current plan to move it to WTF (as > > by Chris Marrin suggested back in > > http://www.mail-archive.com/webkit-dev@lists.webkit.org/msg12672.html), > > clean up the code further, and investigate wider deployment (like to > > the DOM tree) for the security benefit and possible perf win. > > https://bugs.webkit.org/show_bug.cgi?id=101087 > > Having dealt with the specific technical question of RenderArena, I'd like > to briefly discuss the meta-level of how the WebKit project works. > > Sam Weinig and I both provided review feedback saying that using > RenderArena more was a bad idea > (https://bugs.webkit.org/show_bug.cgi?id<https://bugs.webkit.org/show_bug.cgi?id=101087#c9> *This seems completely unfair * * > * *Geoff* * > * *_______________________________________________* *webkit-dev mailing list* *webkit-dev@lists.webkit.org* *http://lists.webkit.org/mailman/listinfo/webkit-dev* =101087#c9 <https://bugs.webkit.org/show_bug.cgi?id=101087#c9>, > https://bugs.webkit.org/show_bug.cgi?id=101087#c18). Nonetheless, you > r+ed a patch to move in that direction, and you describe it here as the > "current plan" for WebKit. > > I'm a little disappointed that, as individual contributors, Chris Neklar > and Chris Evans didn't realize or understand the problems listed above, and > didn't tackle them. However, the mistake is understandable: Chris and Chris > are new to WebKit. The WebKit project has a mechanism for resolving > mistakes like this: patch review. > > Your job as a reviewer is to understand the zeitgeist of the project, to > use good judgement, and to r- patches that make mistakes like this. A bad > patch is only a small nuisance. But the small nuisance turns into a major > problem when you, as a reviewer, take a bad patch, mark it r+, and declare > it the current direction of the project, despite the objections of two > other reviewers who are senior members of the project. > As someone outside all these discussions, this seems like a completely unfair characterization of what happened. Sam voiced an objection, then there was a bunch of discussion in which Chris made an argument that Eric found compelling. Many days passed, then Eric r+'ed. Unless there was other discussion not on the bug that I missed, your objection came after Eric's r+. I don't see the process problem here.
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