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Re: LMDB: issue with mdb_cursor_del
On Wed, 2017-10-18 at 06:18 +0200, Hallvard Breien Furuseth wrote:
> On 16/10/17 14:56, Hallvard Breien Furuseth wrote:
> > On 16. okt. 2017 13:43, Леонид Юрьев wrote:
> > > (...)
> > > Inside freelist_save() the mt_loose_pages could be merged into
> > > the
> > > mt_free_pgs.
> > > http://www.openldap.org/devel/gitweb.cgi?p=openldap.git;a=blob;f=
> > > libraries/liblmdb/mdb.c;h=1bf81ed4c87901991b7cac885b0d8cbc4dd2343
> > > 6;hb=bb8502f08800a44a6b91a94d6478aa7101c4cc77#l3444
> > >
> > >
> > > But at this point all loose_pages also still present in the
> > > mt_u.dirty_list.
> > > (...)
> >
> > I don't understand you. page_alloc doesn't call mdb_mid2l_insert()
> > for loose pages, since they are in dirty_list.
>
> Duh, never mind, I was reading your explanation backwards.
>
> So, when freelist_save() returns loose pages to mt_free_pgs(), you
> remove them from mt_u.dirty_list and update dirty_room. OK.
>
> Howard, I accidentally suggested a different fix to this recently.
> Maybe in LMDB 1.0 we should switch to this:
>
> Set INITIAL_TXNID > 2, so freelist_save() always has an unused freeDB
> key < txnid which it can back up to. Now allow ovpage_free() and
> freelist_save() to create me_pghead. That means finding a nonzero
> value to set me_pglast to as well, or removing the assumtion that
> me_phead == NULL if me_pglast == 0.
>
> Discussed on irc on Aug 10 and rejected, but it may be cleaner
> after all - freelist_save() can quit messing with loose pages
> instead of adding to the code which does that.
> Unless we also rejected it because it wouldn't work:-)
>
Maybe mdb_page_dirty could be allowed to simply accept -1 as a return
value from insert() and that could fix it? Since the page is already
listed in the txn's dirty list, it shouldn't really matter. Or am I
misunderstanding the consequences of this?